Edgar Filing: ARCH CAPITAL GROUP LTD.

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Filed Pursuant to Rule 424(B)(2)
Registration No. 333-117099

P R O S P E C T U S S U P P L E M E N T
(to prospectus dated August 25, 2004)

8,000,000 Shares

8.00% Non-Cumulative Preferred Shares, Series A

Arch Capital Group Ltd. is offering 8,000,000 shares of its 8.00% Non-Cumulative Preferred Shares, Series A, $25 liquidation preference per share (the "Series A Non-Cumulative Preferred Shares").

Holders of Series A Non-Cumulative Preferred Shares will be entitled to receive dividend payments only when, as and if declared by our board of directors or a duly authorized committee of the board. Any such dividends will be payable from the date of original issue on a non-cumulative basis, quarterly in arrears on the 15th day of February, May, August and November of each year (each, a "dividend payment date"), commencing on May 15, 2006, at an annual rate of 8.00%.

Dividends on the Series A Non-Cumulative Preferred Shares are not cumulative. Accordingly, in the event dividends are not declared on the Series A Non-Cumulative Preferred Shares for payment on any dividend payment date, then those dividends will not accrue and will not be payable. If we have not declared a dividend before the dividend payment date for any dividend period, we will have no obligation to pay dividends for that dividend period, whether or not dividends on the Series A Non-Cumulative Preferred Shares are declared for any future dividend period.

So long as any Series A Non-Cumulative Preferred Shares remain outstanding, no dividend shall be paid or declared on our common shares or any of our other securities ranking junior to the Series A Non-Cumulative Preferred Shares (other than a dividend payable solely in common shares or in such other junior securities), unless the full dividends for the latest completed dividend period on all outstanding Series A Non-Cumulative Preferred Shares and any parity shares have been declared and paid or provided for. In the past, we have not paid dividends on our common shares.

Except in specified circumstances relating to certain tax or corporate events, the Series A Non-Cumulative Preferred Shares are not redeemable prior to February 1, 2011. On and after that date, the Series A Non-Cumulative Preferred Shares will be redeemable at our option, in whole or in part, at a redemption price of $25 per Series A Non-Cumulative Preferred Share, plus any declared and unpaid dividends, without accumulation of any undeclared dividends. See "Description of the Series A Non-Cumulative Preferred Shares—Redemption" and "—Tax Redemption." Our ability to redeem the Series A Non-Cumulative Preferred Shares is subject to certain restrictions described under "Description of the Series A Non-Cumulative Preferred Shares—Redemption."

The Series A Non-Cumulative Preferred Shares will not have voting rights, except as set forth under "Description of the Series A Non-Cumulative Preferred Shares—Voting Rights."

See "Risk Factors" beginning on page S-9 of this prospectus supplement and on page 2 of the accompanying prospectus to read about important factors you should consider before investing in the Series A Non-Cumulative Preferred Shares.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus supplement or the accompanying prospectus are truthful or complete. Any representation to the contrary is a criminal offense.

	Per Series A Preferred Share		Total
Public offering price	$	25.0000	$	200,000,000
Underwriting discount	$	0.7875	$	6,300,000
Proceeds, before expenses, to ACGL	$	24.2125	$	193,700,000

We intend to apply to have the Series A Non-Cumulative Preferred Shares approved for listing on the New York Stock Exchange ("NYSE").

The underwriters expect to deliver the Series A Non-Cumulative Preferred Share in book entry form only, through the facilities of The Depositary Trust Company against payment in New York, New York on or about February 1, 2006.

Joint Book-Running Managers

Merrill Lynch & Co.

JPMorgan

Citigroup

Goldman, Sachs & Co.

UBS Investment Bank

Wachovia Securities

Credit Suisse

The date of this prospectus supplement is January 25, 2006.

TABLE OF CONTENTS

		Page
Prospectus Supplement
Prospectus Supplement Summary		S-1
Risk Factors		S-9
Use of Proceeds		S-28
Ratio of Earnings to Fixed Charges and Preference Share Dividends		S-29
Capitalization		S-30
Description of the Series A Non-Cumulative Preferred Shares		S-31
Certain Income Tax Considerations		S-42
Underwriting		S-50
Legal Matters		S-55
Experts		S-55
Cautionary Note Regarding Forward-Looking Statements		S-56
Where You Can Find Additional Information		S-57

		Page
Prospectus
Arch Capital Group Ltd.		1
Arch Capital Group (U.S.) Inc.		1
Risk Factors.		2
Ratio of Earnings to Fixed Charges and Preference Share Dividends		20
Use of Proceeds		21
General Description of the Offered Securities		21
Description of Arch Capital Share Capital		21
Description of Arch Capital Common Shares		22
Description of Arch Capital Preference Shares		23
Description of Arch Capital Debt Securities		26
Description of Arch Capital Depositary Shares		40
Description of Arch Capital Warrants to Purchase Common Shares or Preference Shares		43
Description of Arch Capital Warrants to Purchase Debt Securities		45
Description of Arch Capital Share Purchase Contracts and Share Purchase Units		46
Description of Arch Capital Group (U.S.) Senior Debt Securities		47
Plan of Distribution		60
Where You Can Find More Information		62
Incorporation of Documents by Reference		63
About This Prospectus		64
Cautionary Note Regarding Forward-Looking Statements		64
Legal Matters		66
Experts		66
Enforcement of Civil Liabilities Under United States Federal Securities Laws		67

You should rely only on the information contained or incorporated by reference in this prospectus supplement and the accompanying prospectus. We have not, and the underwriters have not, authorized any other person to provide you with different information. If anyone provides you with different or inconsistent information, you should not rely on it. We are not, and the underwriters are not, making an offer to sell these securities in any jurisdiction where the offer or sale is not permitted. You should assume that the information appearing in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein and in the accompanying prospectus is accurate only as of their respective dates. Our business, financial condition, results of operations and prospects may have changed since those dates.

In this prospectus supplement and in the accompanying prospectus, unless the context requires otherwise: (a) "ACGL" and "Arch Capital" refer to Arch Capital Group Ltd., (b) "we," "us," "our" and "the Company" refer to ACGL and its subsidiaries, (c) "Arch Re Bermuda" refers only to our wholly owned Bermuda reinsurance subsidiary, Arch Reinsurance Ltd., (d) "Arch Re U.S." refers only to our wholly owned U.S. reinsurance subsidiary, Arch Reinsurance Company, (e) (i) "Arch Specialty" refers to Arch Specialty Insurance Company, (ii) "Arch E&S" refers to Arch Excess & Surplus Insurance Company and (iii) "Western Diversified" refers to Western Diversified Casualty Insurance Company, which along with Arch Insurance Company ("Arch Insurance") are our wholly owned U.S. insurance subsidiaries, and (f) "Arch-Europe" refers to our wholly-owned U.K. insurance subsidiary, Arch Insurance Company (Europe) Ltd. The terms "Warburg Pincus funds" and "Hellman & Friedman funds" refer to investment funds affiliated with Warburg Pincus LLC and investment funds led by Hellman & Friedman LLC, respectively.

Securities may be offered or sold in Bermuda only in compliance with the provisions of the Investment Business Act of 2003 of Bermuda, which regulates the sale of securities in Bermuda. In addition, the Bermuda Monetary Authority (the "BMA") must approve all issuances and transfers of securities of a Bermuda exempted company. The BMA has issued its permission for the free issuance and transferability of our securities, as long as any of our shares are listed on an appointed stock exchange, which includes the NYSE, to and among persons who are non-residents of Bermuda for exchange control purposes. The issue and transfer of in excess of 20% of the common shares to and among persons who are residents of Bermuda for exchange control purposes requires prior authorization from the BMA. Any other transfers remain subject to approval by the BMA. In addition, this prospectus supplement and the accompanying prospectus will be filed with the Registrar of Companies in Bermuda in accordance with Bermuda law. The BMA and the Registrar of Companies accept no responsibility for the financial soundness of any proposal or for the correctness of any of the statements made or opinions expressed in this prospectus supplement or in the accompanying prospectus.

PROSPECTUS SUPPLEMENT SUMMARY

This summary is not complete and does not contain all the information you should consider. You should read this entire prospectus supplement and the accompanying prospectus carefully, including without limitation, the documents incorporated by reference in this prospectus supplement or the accompanying prospectus, the sections entitled "Risk Factors" in this prospectus supplement and in the accompanying prospectus and the section entitled "Cautionary Note Regarding Forward-Looking Statements." Unless otherwise stated, the information presented in this prospectus supplement (not including the documents incorporated herein by reference) gives effect to the conversion of the remaining outstanding series A convertible preference shares into a like number of our common shares, which occurred in the fourth quarter of 2005. See "Recent Developments" below and "Description of Arch Capital Preference Shares- Series A Convertible Preference Shares" in the accompanying prospectus.

Our Company

Arch Capital Group Ltd. is a Bermuda public limited liability company with $2.65 billion in capital as of September 30, 2005 and, through operations in Bermuda, the United States, Europe and Canada, writes insurance and reinsurance on a worldwide basis. While we are positioned to provide a full range of property and casualty insurance and reinsurance lines, we focus on writing specialty lines of insurance and reinsurance. It is our belief that our underwriting platform, our experienced management team and our strong capital base that is unencumbered by significant pre-2002 risks have enabled us to establish a strong presence in an attractive insurance and reinsurance marketplace.

The worldwide insurance and reinsurance industry is highly competitive and has traditionally been subject to an underwriting cycle in which a hard market (high premium rates, restrictive underwriting standards, as well as terms and conditions, and underwriting gains) is eventually followed by a soft market (low premium rates, relaxed underwriting standards, as well as terms and conditions, and underwriting losses). Insurance market conditions may affect, among other things, the demand for our products, our ability to increase premium rates, the terms and conditions of the insurance policies we write, changes in the products offered by us or changes in our business strategy.

The financial results of the insurance and reinsurance industry are influenced by factors such as the frequency and/or severity of claims and losses, including natural disasters or other catastrophic events, variations in interest rates and financial markets, changes in the legal, regulatory and judicial environments, inflationary pressures and general economic conditions. These factors influence, among other things, the demand for insurance or reinsurance, the supply of which is generally related to the total capital of competitors in the market. During 2001, market conditions had been improving primarily as a result of declining insurance capacity.

In general, market conditions improved during 2002 and 2003 in the insurance and reinsurance marketplace. This reflected improvement in pricing, terms and conditions following significant industry losses arising from the events of September 11th, as well as the recognition that intense competition in the late 1990s led to inadequate pricing and overly broad terms, conditions and coverages. Such industry developments resulted in poor financial results and erosion of the industry's capital base. Consequently, many established insurers and reinsurers reduced their participation in, or exited from, certain markets and, as a result, premium rates escalated in many lines of business. These developments provided relatively new insurers and reinsurers, like us, with an opportunity to provide needed underwriting capacity. Beginning in late 2003 and continuing through 2005, additional capacity emerged in many classes of business and, consequently, premium rate increases have decelerated significantly and, in many classes of business, premium rates have decreased. However, we believe that we are still able to write insurance and reinsurance business at what we believe to be attractive rates. In addition, the catastrophic events that occurred in the second half of 2005 have resulted in a substantial improvement in market conditions in property and certain marine lines of business. The

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actual impact of such events on premium rates and other market conditions should become more apparent over the next several months.

We monitor our capital adequacy on a regular basis and will seek to adjust our capital base (up or down) according to the needs of our business. The future capital requirements of our business will depend on many factors, including our ability to write new business successfully and to establish premium rates and reserves at levels sufficient to cover losses. Our ability to underwrite is largely dependent upon the quality of our claims paying and financial strength ratings as evaluated by independent rating agencies. In particular, we require (1) sufficient capital to maintain our financial strength ratings, as issued by several ratings agencies, at a level considered necessary by management to enable our key operating subsidiaries to compete; (2) sufficient capital to enable our underwriting subsidiaries to meet the capital adequacy tests performed by statutory agencies in the U.S. and other key markets; and (3) letters of credit and other forms of collateral that are required by our non-U.S. operating companies that are "non-admitted" under U.S. state insurance regulations.

As part of our capital management program, we may seek to raise additional capital or may seek to return capital to our shareholders through share repurchases, cash dividends or other methods (or a combination of such methods). Any such determination will be at the discretion of our board of directors and will be dependent upon our profits, financial requirements and other factors, including legal restrictions, rating agency requirements and such other factors as our board of directors deems relevant.

Recent Developments

Conversion of series A convertible preference shares

On December 30, 2005, the holders of 32,327,502 of our series A convertible preference shares converted all of such shares into an equal number of our common shares pursuant to the certificate of designations relating to the series A convertible preference shares. Following such conversion, there are no remaining outstanding series A convertible preference shares. Since the preference shares were treated as common share equivalents in our reported financial results, the conversion will have no effect on diluted net income per share or diluted book value per share in future periods.

Flatiron Retrocession Facility

On December 29, 2005, Arch Re Bermuda (the "Ceding Company") entered into a quota share reinsurance treaty (the "Treaty") with Flatiron Re Ltd. ("Flatiron Re"), a newly-formed Bermuda reinsurance company, pursuant to which Flatiron Re is assuming a 45% quota share of certain lines of property and marine business underwritten by the Ceding Company for the 2006 and 2007 underwriting years (commencing January 1, 2006 through and including December 31, 2007). The quota share is subject to decrease by the Ceding Company under certain circumstances. In addition, in certain circumstances, Flatiron Re may extend at its option the coverage provided by the Treaty to the Ceding Company's 2008 underwriting year.

Flatiron Re is required to contribute funds into a trust for the benefit of the Ceding Company (the "Trust"). Under the Treaty, the amount required to be on deposit in the Trust, together with certain other amounts, is a calculated amount estimated to cover ceded losses arising from in excess of two 1-in-250 year events for the applicable forward twelve-month period (the "Requisite Funded Amount"). During the underwriting years covered by the Treaty, the Requisite Funded Amount will be subject to a minimum dollar level that will increase to $900,000,000 by July 1, 2006. If the actual amounts on deposit in the Trust, together with certain other amounts (the "Funded Amount"), do not at least equal the Requisite Funded Amount, the Ceding Company will, among other things, reduce the percentage of business ceded on a prospective basis and, at the Ceding Company's option, recapture unearned premium reserves and reassume losses that would have been ceded in respect of such unearned premiums. No assurances can be given that actual losses will not exceed the Requisite Funded Amount

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or that Flatiron Re will make, or will have the ability to make, the required contributions into the Trust.

The Ceding Company will pay to Flatiron Re a reinsurance premium in the amount of the ceded percentage of the original gross written premium on the business reinsured with Flatiron Re less a ceding commission, which includes a reimbursement of direct acquisition expenses as well as a commission to the Ceding Company for generating the business. The Treaty also provides for a profit commission to the Ceding Company based on the underwriting results for the 2006 and 2007 underwriting years on a cumulative basis.

The Ceding Company will have the right to terminate its obligations to cede business to Flatiron Re if, among other things, the assets held in trust for its benefit do not meet certain conditions, if ceded unpaid loss reserves equal or exceed the Funded Amount or if the direct or indirect ownership of Flatiron Re changes in certain respects.

Closing of new revolving loan and letter of credit facilities

As previously announced, on November 29, 2005, we entered into new revolving loan and letter of credit facilities (which we sometimes refer to as the new credit facilities) with a syndicate of banks and JPMorgan Chase Bank, N.A., as administrative agent. The new credit facilities include a five-year $300 million unsecured revolving loan and letter of credit facility and a five-year $500 million secured letter of credit facility. The new credit facilities are an amendment and restatement of our previous revolving loan and letter of credit facilities, which provided for a three-year $300 million unsecured revolving loan and letter of credit facility and a three-year $400 million secured letter of credit facility In addition, as previously announced, on January 18, 2006, Arch Re Bermuda amended and restated its existing secured letter of credit and reimbursement facility, with Barclays Bank PLC, as agent and issuer, to, among other things, increase the maximum face amount of letters of credit available for issuance thereunder from $175 million to $200 million.

Hurricane Wilma

As previously announced, we expect after-tax earnings for the 2005 fourth quarter to be negatively impacted by the effects of Hurricane Wilma in the range of $40 million to $60 million based on industry insured losses of $8 to $12 billion. These preliminary estimates are net of reinsurance and reinstatement premiums. The losses from the storm are currently expected to arise approximately 56% in our insurance operations and approximately 44% in our reinsurance operations if the industry loss is at the lower end of the range and move to approximately a 48/52 split, insurance and reinsurance, if the industry loss is at the upper end of the range.

The estimates are based on currently available information derived from modeling techniques, industry assessments of exposure, preliminary claims information obtained from our clients and brokers to date and a review of our in-force contracts. Our actual losses from this event may vary materially from the estimates due to the inherent uncertainties in making such determinations resulting from several factors, including the preliminary nature of the available information, the potential inaccuracies and inadequacies in the data provided by clients and brokers, the modeling techniques and the application of such techniques, the contingent nature of business interruption exposures, the effects of any resultant demand surge on claims activity and attendant coverage issues. In addition, actual losses may increase if our reinsurers fail to meet their obligations to us or the reinsurance protections purchased by us are exhausted or are otherwise unavailable.

Principal Executive Office

Our registered office is located at Clarendon House, 2 Church Street, Hamilton HM 11, Bermuda (telephone number: (441) 295-1422), and our principal executive offices are located at Wessex House, 45 Reid Street, Hamilton HM 12, Bermuda (telephone number: (441) 278-9250). We maintain a website at http://www.archcapgroup.bm. The information contained on our website is not incorporated herein by reference and does not form a part of this prospectus supplement or the accompanying prospectus.

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THE OFFERING

The following is a brief summary of certain terms of this offering. For a more complete description of the terms of Series A Non-Cumulative Preferred Shares, see "Description of the Series A Non-Cumulative Preferred Shares" in this prospectus supplement.

Issuer		Arch Capital Group Ltd.
Securities offered		8,000,000 shares of 8.00% Non-Cumulative Preferred Shares, Series A (or "Series A Non-Cumulative Preferred Shares"), $0.01 par value per share, with a liquidation preference of $25 per share, of Arch Capital. We may from time to time elect to issue additional Series A Non-Cumulative Preferred Shares, and all the additional shares would be deemed to form a single series with the Series A Non-Cumulative Preferred Shares.
Dividends		Dividends on the Series A Non-Cumulative Preferred Shares, when, as and if declared by the board of directors of Arch Capital or a duly authorized committee of the board, will accrue and be payable on the liquidation preference amount from the original issue date, on a non-cumulative basis, quarterly in arrears on each dividend payment date, at an annual rate of 8.00%. Any such dividends will be distributed to holders of the Series A Non-Cumulative Preferred Shares in the manner described under "Description of the Series A Non-Cumulative Preferred Shares—Dividends" in this prospectus supplement. Dividends will be computed on the basis of a 360-day year consisting of twelve 30-day months. Dividends on the Series A Non-Cumulative Preferred Shares are not cumulative. Accordingly, in the event dividends are not declared on the Series A Non-Cumulative Preferred Shares and any parity shares for payment on any dividend payment date, then such dividends will not accrue and will not be payable. If the board of directors of Arch Capital or a duly authorized committee of the board has not declared a dividend before the dividend payment date for any dividend period, we will have no obligation to pay dividends for such dividend period after the dividend payment date for that dividend period, whether or not dividends on the Series A Non-Cumulative Preferred Shares are declared for any future dividend period.
		We believe that the dividends paid on the Series A Non-Cumulative Preferred Shares should qualify as "qualified dividend income" if, as is intended, the Series A Non-Cumulative Preferred Shares are approved for listing on the NYSE. Qualified dividend income paid before January 1, 2009 to certain shareholders is subject to tax at long-term capital gain rates. Dividends paid on the Series A Non- Cumulative Preferred Shares to U.S. corporate shareholders will not be eligible for the dividends-received deduction. There is a risk that dividends, if any, paid prior to the listing of the Series A Non-Cumulative Preferred Shares on the NYSE may not constitute qualified dividend income. See "Certain Income Tax Considerations—Taxation of Shareholders—United States Taxation—Taxation of Dividends" in this prospectus supplement.

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Dividend Payment Dates	The 15th day of February, May, August and November of each year, commencing on May 15, 2006. If any date on which dividends would otherwise be payable is not a business day, then the dividend payment date will be the next succeeding business day with the same force and effect as if made on the original dividend payment date. Dividends on the Series A Non-Cumulative Preferred Shares will not be mandatory.
Payment of Additional Amounts and Tax Redemption	Subject to certain limitations, we will pay additional amounts to holders of the Series A Non-Cumulative Preferred Shares, as additional dividends, to make up for any deduction or withholding for any taxes or other charges imposed on amounts we must pay with respect to the Series A Non-Cumulative Preferred Shares, so that the net amounts paid will be equal to the amounts we would otherwise be required to pay had no such withholding or deduction been required. See "Description of the Series A Non-Cumulative Preferred Shares—Payment of Additional Amounts" in this prospectus supplement. If we become obligated to pay any additional amounts as a result of a change in tax law, we will also have the option to redeem the Series A Non-Cumulative Preferred Shares, at any time in whole or in part from time to time, at a redemption price of $25 per share plus declared and unpaid dividends, if any, to the date of redemption. See "Description of the Series A Non-Cumulative Preferred Shares—Tax Redemption" in this prospectus supplement.
Redemption	On and after February 1, 2011, the Series A Non-Cumulative Preferred Shares will be redeemable at our option, in whole or in part, at a redemption price equal to $25 per Series A Non-Cumulative Preferred Share, plus any declared and unpaid dividends, without accumulation of any undeclared dividends. We may not redeem the Series A Non-Cumulative Preferred Shares before February 1, 2011, except as described under "Description of the Series A Non-Cumulative Preferred Shares—Tax Redemption" in this prospectus supplement, and except that we may redeem all but not less than all of the Series A Non-Cumulative Preferred Shares before that date at a redemption price of $26 per share, plus any declared and unpaid dividends, to the date of redemption, if we
	•	submit a proposal to our holders of common shares concerning an amalgamation, consolidation, merger, arrangement, reconstruction, reincorporation, de-registration or other similar transaction involving us that requires a vote of the holders of our Series A Non-Cumulative Preferred Shares, voting separately as a single class (alone or with one or more classes or series of preferred shares); or

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	•	submit any proposal for any other matter that, as a result of any change in Bermuda law after the date of this prospectus supplement (whether by enactment or official interpretation), requires a vote of the holders of our Series A Non-Cumulative Preferred Shares, voting separately as a single class (alone or with one or more classes or series of preferred shares).
	Our ability to redeem the Series A Non-Cumulative Preferred Shares as described above or pursuant to certain tax events may be limited by covenants contained in our credit facilities and by the provisions of other agreements we may enter into.
	The Series A Non-Cumulative Preferred Shares will not be subject to any sinking fund or other obligation of ours to redeem, purchase or retire the Series A Non-Cumulative Preferred Shares.
	Under Bermuda law, no redemption of the Series A Non-Cumulative Preferred Shares may be effected if, on the date that the redemption is to be effected, we have reasonable grounds for believing that we are, or after the redemption would be, unable to pay our liabilities as they become due.
Ranking	The Series A Non-Cumulative Preferred Shares:
	•	will rank senior to our junior shares with respect to the payment of dividends and distributions upon our liquidation, dissolution or winding-up. Junior shares includes our common shares and any other class of shares that rank junior to the Series A Non-Cumulative Preferred Shares either as to the payment of dividends or as to the distribution of assets upon liquidation, dissolution or winding-up; and
	•	will rank at least equally with each other series of shares ranking on parity with the Series A Non-Cumulative Preferred Shares as to dividends and distributions upon any liquidation or dissolution or winding-up of Arch Capital, which we refer to as parity shares.
	As of the date of this prospectus supplement we have no outstanding shares or series of shares that are on parity with the Series A Non-Cumulative Preferred Shares with respect to the payment of dividends and the distribution of assets upon a liquidation, dissolution or winding-up of Arch Capital.
	During any dividend period, so long as any Series A Non-Cumulative Preferred Shares remain outstanding, unless the full dividends for the latest completed dividend period on all outstanding Series A Non-Cumulative Preferred Shares have been declared and paid:

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	•	no dividend shall be paid or declared on our common shares or other junior shares, other than a dividend payable solely in junior shares; and
	•	no common shares or other junior shares shall be purchased, redeemed or otherwise acquired for consideration by us, directly or indirectly (other than (i) as a result of a reclassification of junior shares for or into other junior shares, or the exchange or conversion of one junior share for or into another junior share, (ii) through the use of the proceeds of a substantially contemporaneous sale of junior shares and (iii) as permitted by the bye-laws of Arch Capital in effect on the date of issuance of the Series A Non-Cumulative Preferred Shares).
	For any dividend period in which dividends are not paid in full upon the Series A Non-Cumulative Preferred Shares and any parity shares, all dividends declared for such dividend period with respect to the Series A Non-Cumulative Preferred Shares and such parity shares shall be declared on a pro rata basis. See "Description of the Series A Non-Cumulative Preferred Shares—Dividends" in this prospectus supplement.
Liquidation Rights	Upon any voluntary or involuntary liquidation, dissolution or winding-up of Arch Capital, holders of the Series A Non-Cumulative Preferred Shares and any parity shares are entitled to receive out of our assets available for distribution to shareholders, before any distribution is made to holders of common shares or other junior shares, a liquidating distribution in the amount of $25 per Series A Non-Cumulative Preferred Share plus any declared and unpaid dividends, without accumulation of any undeclared dividends. Distributions will be made pro rata as to the Series A Non-Cumulative Preferred Shares and any parity shares and only to the extent of our assets, if any, that are available after satisfaction of all liabilities to creditors. See "Description of the Series A Non-Cumulative Preferred Shares—Liquidation Rights" in this prospectus supplement.
Voting Rights	Holders of the Series A Non-Cumulative Preferred Shares will have no voting rights, except with respect to certain fundamental changes in the terms of the Series A Non-Cumulative Preferred Shares and in the case of certain dividend non-payments or as otherwise required by Bermuda law or the bye-laws of Arch Capital. See "Description of the Series A Non-Cumulative Preferred Shares—Voting Rights" in this prospectus supplement.
Maturity	The Series A Non-Cumulative Preferred Shares do not have any maturity date, and we are not required to redeem the Series A Non-Cumulative Preferred Shares. Holders of the Series A Non-Cumulative Preferred Shares will have no right to have the Series A Non-Cumulative Preferred Shares redeemed. Accordingly, the Series A Non-Cumulative Preferred Shares will remain outstanding indefinitely, unless and until we decide to redeem them.

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Preemptive Rights		Holders of the Series A Non-Cumulative Preferred Shares will have no preemptive rights.
Listing		We intend to apply to have the Series A Non-Cumulative Preferred Shares approved for listing on the NYSE.
Ratings		The Series A Non-Cumulative Preferred Shares are rated BB+ by Standard & Poor's, Baa3 by Moody's Investors Service and BBB- by Fitch Ratings. The ratings of the Series A Non-Cumulative Preferred Shares should be evaluated independently from similar ratings of other securities. A rating is not a recommendation to buy, sell or hold securities and may be subject to review, revision, suspension, reduction or withdrawal at any time by the assigning rating agency.
Use of proceeds		We expect to receive approximately $192 million in net proceeds from the sale of the Series A Non-Cumulative Preferred Shares in this offering. We intend to use the net proceeds of this offering for general corporate purposes.
Transfer Agent		American Stock Transfer & Trust Company.

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RISK FACTORS

An investment in the Series A Non-Cumulative Preferred Shares involves the following risks. You should consider carefully these risk factors and also refer to the other information provided in this prospectus supplement and the accompanying prospectus and the documents incorporated therein by reference, including our "Management's Discussion and Analysis of Financial Condition and Results of Operation" and our financial statements and the related notes which are included in our Annual Report on Form 10-K for the year ended December 31, 2004 and our Quarterly Report on Form 10-Q for the quarter ended September 30, 2005, which is incorporated by reference in this prospectus supplement, as well as the information under the heading "Cautionary Note Regarding Forward-Looking Statements."

Risks Relating to Our Industry

We operate in a highly competitive environment and we may not be able to compete successfully in our industry.

The insurance and reinsurance industry is highly competitive. We compete with major U.S. and non-U.S. insurers and reinsurers, many of which have greater financial, marketing and management resources than we do. We also compete with new companies that continue to be formed to enter the insurance and reinsurance markets. In our insurance business, we compete with insurers that provide property and casualty lines of insurance, including ACE Limited, Allied World Assurance Company, Ltd., American International Group, Inc., AXIS Capital Holdings Limited, Berkshire Hathaway, Inc., Chubb Corporation, Endurance Specialty Holdings Ltd., The Hartford Financial Services Group, Inc., Lloyd's of London, The St. Paul Travelers Companies, XL Capital Ltd. and Zurich Insurance Group. In our reinsurance business, we compete with reinsurers that provide property and casualty lines of reinsurance, including ACE Limited, AXIS Capital Holdings Limited, Converium Group, Endurance Specialty Holdings Ltd., Everest Re Group Ltd., General Reinsurance Corporation, Hannover Rückversicherung AG, Lloyd's of London, Montpelier Re Holdings Ltd., Munich Re Group, PartnerRe Ltd., Platinum Underwriters Holdings, Ltd., RenaissanceRe Holdings Ltd., Swiss Reinsurance Company, Transatlantic Holdings, Inc. and XL Capital Ltd. We do not believe that we have a significant market share in any of our markets.

Trends toward consolidation in the insurance industry could also lead to pricing pressure and lower margins for insurers and reinsurers. In addition, since the recent hurricanes in the second half of 2005 and other events, several newly formed offshore entities are entering the insurance and reinsurance markets. Several publicly traded insurance and reinsurance companies have also raised additional capital to meet perceived demand in the current environment. Financial institutions and other capital markets participants also offer alternative products and services similar to our own or alternative products that compete with insurance and reinsurance products. In addition, we may not be aware of other companies that may be planning to enter the segments of the insurance and reinsurance market in which we operate or of existing companies that may be planning to raise additional capital.

Our competitive position is based on many factors, including our perceived overall financial strength, ratings assigned by independent rating agencies, geographic scope of business, client relationships, premiums charged, contract terms and conditions, products and services offered (including the ability to design customized programs), speed of claims payment, reputation, experience and qualifications of employees and local presence. We may not be successful in competing with others in our industry on any of these bases, and the intensity of competition in our industry may erode profitability and result in less favorable policy terms and conditions for insurance and reinsurance companies generally, including us.

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The insurance and reinsurance industry is highly cyclical, and we expect to experience periods characterized by excess underwriting capacity and unfavorable premium rates.

Historically, insurers and reinsurers have experienced significant fluctuations in operating results due to competition, frequency of occurrence or severity of catastrophic events, levels of capacity, general economic conditions, changes in legislation, case law and prevailing concepts of liability and other factors. In particular, demand for reinsurance is influenced significantly by the underwriting results of primary insurers and prevailing general economic conditions. The supply of insurance and reinsurance is related to prevailing prices and levels of surplus capacity that, in turn, may fluctuate in response to changes in rates of return being realized in the insurance and reinsurance industry. As a result, the insurance and reinsurance business historically has been a cyclical industry characterized by periods of intense price competition due to excessive underwriting capacity as well as periods when shortages of capacity permitted favorable premium levels and changes in terms and conditions.

We could face unanticipated losses from war, terrorism and political unrest, and these or other unanticipated losses could have a material adverse effect on our financial condition and results of operations.

Although we believe that we do not have exposure to the events of September 11, 2001 because we did not have insurance in-force at that time with respect to exposure to such events, we now have substantial exposure to unexpected, large losses resulting from future man-made catastrophic events, such as acts of war, acts of terrorism and political instability. These risks are inherently unpredictable, although recent events may lead to increased frequency and severity of losses. It is difficult to predict the timing of such events with statistical certainty or estimate the amount of loss any given occurrence will generate. In certain instances, we specifically insure and reinsure risks resulting from acts of terrorism. Even in cases where we attempt to exclude losses from terrorism and certain other similar risks from some coverages written by us, we may not be successful in doing so. Moreover, irrespective of the clarity and inclusiveness of policy language, there can be no assurance that a court or arbitration panel will limit enforceability of policy language or otherwise issue a ruling adverse to us. Accordingly, while we believe our reinsurance programs, together with the coverage provided under the Terrorism Risk Insurance Act of 2002, as amended under the Terrorism Risk Insurance Extension Act of 2005 ("TRIA"), are sufficient to reasonably limit our net losses relating to potential future terrorist attacks, we can offer no assurance that our reserves will be adequate to cover losses when they materialize. To the extent that an act of terrorism is certified by the Secretary of the Treasury, our U.S. insurance operations may be covered under TRIA for up to 90% of its losses for 2005 and 2006 and up to 85% of its losses for 2007, in each case subject to certain mandatory deductibles. See "Our Company—Regulation—U.S. Insurance Regulation" contained in our Annual Report on Form 10-K for the year ended December 31, 2004 incorporated here by reference. It is not possible to eliminate completely our exposure to unforecasted or unpredictable events, and to the extent that losses from such risks occur, our financial condition and results of operations could be materially adversely affected.

Claims for catastrophic events could cause large losses and substantial volatility in our results of operations, and, as a result, the value of our securities, including the Series A Non-Cumulative Preferred Shares, may fluctuate widely.

We have large aggregate exposures to natural disasters. Catastrophes can be caused by various events, including hurricanes, floods, windstorms, earthquakes, hailstorms, tornados, explosions, severe winter weather, fires and other natural disasters. Catastrophes can also cause losses in non-property business such as workers' compensation or general liability. In addition to the nature of the property business, we believe that economic and geographic trends affecting insured property, including inflation, property value appreciation and geographic concentration tend to generally increase the size of losses from catastrophic events over time. Our previously disclosed estimates relating to Hurricanes Katrina,

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Rita and Wilma and other catastrophic events that occurred in the second half of 2005 are based on currently available information derived from modeling techniques, industry assessments of exposure, preliminary claims information obtained from our clients and brokers to date and a review of our in-force contracts. Our actual losses from these events may vary materially from our estimates due to the inherent uncertainties in making such determinations resulting from several factors, including the preliminary nature of the available information, the potential inaccuracies and inadequacies in the data provided by clients and brokers, the modeling techniques and the application of such techniques, the contingent nature of business interruption exposures, the effects of any resultant demand surge on claims activity and attendant coverage issues. In addition, actual losses may increase if our reinsurers fail to meet their obligations to us or the reinsurance protections purchased by us are exhausted or are otherwise unavailable.

The catastrophic events that occurred in the second half of 2005 have resulted in a substantial improvement in market conditions in property and certain marine lines of business. In order to take advantage of these opportunities, we are seeking to increase our writings in the property and marine lines of business and, as a result, these lines may represent a larger proportion of our overall book of business in future periods.

In addition, over the past several years, changing weather patterns and climactic conditions, such as global warming, have added to the unpredictability and frequency of natural disasters in certain parts of the world and created additional uncertainty as to future trends and exposures. Furthermore, some experts have attributed the recent high incidence of hurricanes in the Gulf of Mexico and the Caribbean to a permanent change in weather patterns resulting from rising surface ocean temperature in the region (the "warm ocean" theory). Therefore, claims for catastrophic events could expose us to large losses and cause substantial volatility in our results of operations, which could cause the value of our securities, including the Series A Non-Cumulative Preferred Shares, to fluctuate widely. Our ability to write new business could also be adversely impacted.

Underwriting claims and reserving for losses are based on probabilities and related modeling, which are subject to inherent uncertainties.

Our success is dependent upon our ability to assess accurately the risks associated with the businesses that we insure and reinsure. We establish reserves for losses and loss adjustment expenses which represent estimates involving actuarial and statistical projections, at a given point in time, of our expectations of the ultimate settlement and administration costs of losses incurred. We utilize actuarial models as well as available historical insurance industry loss ratio experience and loss development patterns to assist in the establishment of loss reserves. Actual losses and loss adjustment expenses paid will deviate, perhaps substantially, from the reserve estimates reflected in our financial statements.

If our loss reserves are determined to be inadequate, we will be required to increase loss reserves at the time of such determination with a corresponding reduction in our net income in the period in which the deficiency becomes known. It is possible that claims in respect of events that have occurred could exceed our claim reserves and have a material adverse effect on our results of operations, in a particular period, or our financial condition in general. As a compounding factor, although most insurance contracts have policy limits, the nature of property and casualty insurance and reinsurance is such that losses can exceed policy limits for a variety of reasons and could significantly exceed the premiums received on the underlying policies, thereby further adversely affecting our financial condition.

As of September 30, 2005, our reserves for unpaid losses and loss adjustment expenses, net of unpaid losses and loss adjustment expenses recoverable, were approximately $3.86 billion. Such reserves were established in accordance with applicable insurance laws and generally accepted accounting principles ("GAAP"). Although we believe we have applied a conservative reserving philosophy for

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both our insurance and reinsurance operations, insurance loss reserves are inherently subject to uncertainty. In establishing the reserves for losses and loss adjustment expenses, we have made various assumptions relating to the pricing of our reinsurance contracts and insurance policies and have also considered available historical industry experience and current industry conditions. Our reserving process reflects that there is a possibility that the assumptions made could prove to be inaccurate due to several factors, including the fact that limited historical information has been reported to us through September 30, 2005. See Development of GAAP Reserves table included under "Business—Reserves" contained in our Annual Report on Form 10-K for the year ended December 31, 2004 incorporated herein by reference.

The failure of any of the loss limitation methods we employ could have a material adverse effect on our financial condition or results of operations.

We seek to limit our loss exposure by writing a number of our reinsurance contracts on an excess of loss basis, adhering to maximum limitations on reinsurance written in defined geographical zones, limiting program size for each client and prudent underwriting of each program written. In the case of proportional treaties, we may seek per occurrence limitations or loss ratio caps to limit the impact of losses from any one or series of events. We cannot be certain that any of these loss limitation methods will be effective. We also seek to limit our loss exposure by geographic diversification. Geographic zone limitations involve significant underwriting judgments, including the determination of the area of the zones and the inclusion of a particular policy within a particular zone's limits. There can be no assurance that various provisions of our policies, such as limitations or exclusions from coverage or choice of forum, will be enforceable in the manner we intend. Disputes relating to coverage and choice of legal forum may also arise. Underwriting is inherently a matter of judgment, involving important assumptions about matters that are inherently unpredictable and beyond our control, and for which historical experience and probability analysis may not provide sufficient guidance. One or more catastrophic or other events could result in claims that substantially exceed our expectations, which could have a material adverse effect on our financial condition or our results of operations, possibly to the extent of eliminating our shareholders' equity.

For our natural catastrophe exposed business, we seek to limit the amount of exposure we will assume from any one insured or reinsured and the amount of the exposure to catastrophe losses in any geographic zone. We monitor our exposure to catastrophic events, including earthquake and wind and periodically reevaluate the estimated probable maximum pre-tax loss for such exposures. Our estimated probable maximum pre-tax loss is determined through the use of modeling techniques, but such estimate does not represent our total potential loss for such exposures. We seek to limit the probable maximum pre-tax loss to a specific level for severe catastrophic events. Currently, we generally seek to limit the probable maximum pre-tax loss to approximately 25% of total shareholders' equity for a severe catastrophic event in any geographic zone that could be expected to occur once in every 250 years. There can be no assurances that we will not suffer pre-tax losses greater than 25% of our total shareholders' equity from one or more catastrophic events due to several factors, including the inherent uncertainties in estimating the frequency and severity of such events and the margin of error in making such determinations resulting from potential inaccuracies and inadequacies in the data provided by clients and brokers, the modeling techniques and the application of such techniques. In addition, depending on business opportunities and the mix of business that may comprise our insurance and reinsurance portfolio, we may seek to adjust our self-imposed limitations on probable maximum pre-tax loss for catastrophe exposed business.

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The risk associated with reinsurance underwriting could adversely affect us, and while reinsurance and retrocessional coverage will be used to limit our exposure to risks, the availability of such arrangements may be limited, and counterparty credit and other risks associated with our reinsurance arrangements may result in losses which could adversely affect our financial condition and results of operations.

Like other reinsurers, we do not separately evaluate each of the individual risks assumed under reinsurance treaties. Therefore, we are largely dependent on the original underwriting decisions made by ceding companies. We are subject to the risk that the ceding companies may not have adequately evaluated the risks to be reinsured and that the premiums ceded may not adequately compensate us for the risks we assume.

For the purposes of limiting our risk of loss, we use reinsurance and also may use retrocessional arrangements. In the normal course of business, our insurance subsidiaries cede a substantial portion of their premiums through pro rata, excess of loss and facultative reinsurance agreements. Our reinsurance subsidiaries purchase a limited amount of retrocessional coverage as part of their aggregate risk management program. In addition, our reinsurance subsidiaries participate in "common account" retrocessional arrangements for certain pro rata treaties. Such arrangements reduce the effect of individual or aggregate losses to all companies participating on such treaties, including the reinsurers, such as our reinsurance subsidiaries, and the ceding company. For the nine months ended September 30, 2005, ceded premiums written represented approximately 22.2% of gross premiums written, compared to 18.8% and 15.1%, respectively, for the years ended December 31, 2004 and 2003.

The availability and cost of reinsurance and retrocessional protection is subject to market conditions, which are beyond our control. Although we believe that our insurance subsidiaries have been successful in obtaining reinsurance protection since the commencement of our underwriting initiative in October 2001, it is not certain that we will be able to continue to obtain adequate protection at cost effective levels. As a result of such market conditions and other factors, we may not be able successfully to mitigate risk through reinsurance and retrocessional arrangements.

Further, we are subject to credit risk with respect to our reinsurance and retrocessions because the ceding of risk to reinsurers and retrocessionaires does not relieve us of our liability to the clients or companies we insure or reinsure. Our failure to establish adequate reinsurance or retrocessional arrangements or the failure of our existing reinsurance or retrocessional arrangements to protect us from overly concentrated risk exposure could adversely affect our financial condition and results of operations. We monitor the financial condition of our reinsurers and attempt to place coverages only with carriers we view as substantial and financially sound. At September 30, 2005, approximately 82.8% of our reinsurance recoverables on paid and unpaid losses of $1.27 billion (not including prepaid reinsurance premiums) were due from carriers which had an A.M. Best rating of "A-" or better. No reinsurance recoverables from any one carrier exceeded 5.3% of our total shareholders' equity at September 30, 2005. In connection with our acquisition of Arch Specialty in February 2002, the seller, Sentry Insurance a Mutual Company ("Sentry"), agreed to reinsure and guarantee all liabilities arising out of Arch Specialty's business prior to the closing of the acquisition. In addition to the guarantee provided by Sentry, substantially all of the recoverable from Sentry is still subject to the original reinsurance agreements inuring to Arch Specialty and, to the extent Sentry fails to comply with its payment obligations to us, we may obtain reimbursement from the third party reinsures under such agreements.

Our reliance on brokers subjects us to their credit risk.

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make such payment, we may remain liable to the insured or ceding insurer for the deficiency. Likewise, in certain jurisdictions, when the insured or ceding company pays premium for these contracts to brokers for payment to us, these premiums are considered to have been paid and the insured or ceding company will no longer be liable to us for those amounts, whether or not we have actually received the premiums from the broker. Consequently, we assume a degree of credit risk associated with our brokers. To date, we have not experienced any losses related to this credit risk.

We cannot predict the effect that the investigation currently being conducted by the New York Attorney General and others will have on the industry or our business.

The New York Attorney General and others are investigating allegations relating to contingent commission payments, bid-rigging and other practices in the insurance industry. We cannot predict the effect that this investigation, and any changes in insurance practice, including future legislation or regulations that may become applicable to us, will have on the insurance industry, the regulatory framework or our business. See "—Our reliance on brokers subjects us to their credit risk." See also "Management's Discussion and Analysis of Financial Condition and Results of Operations—General—Overview" contained in our Annual Report on Form 10-K for the year ended December 31, 2004 incorporated herein by reference and contained in our Quarterly Report Form 10-Q for the quarter ended September 30, 2005 incorporated herein by reference and "Our Company—Regulation—U.S. Insurance Regulation—General" contained in our Annual Report on Form 10-K for the year ended December 31, 2004 incorporated herein by reference.

The effects of emerging claims and coverage issues are uncertain. The insurance industry is also affected by political, judicial and legal developments which have in the past resulted in new or expanded theories of liability. These or other changes could impose new financial obligations on us by extending coverage beyond our underwriting intent or otherwise require us to make unplanned modifications to the products and services that we provide, or cause the delay or cancellation of products and services that we provide. In some instances, these changes may not become apparent until some time after we have issued insurance or reinsurance contracts that are affected by the changes. As a result, the full extent of liability under our insurance or reinsurance contracts may not be known for many years after a contract is issued. The effects of unforeseen developments or substantial government intervention could adversely impact our ability to achieve our goals.

In addition, we engage in intercompany reinsurance arrangements between our U.S. operations and our Bermuda reinsurance operations. Some U.S. insurance companies have been lobbying Congress to pass legislation intended to eliminate certain perceived tax advantages of U.S. insurance companies with Bermuda affiliates, which result principally from reinsurance arrangements between or among U.S. insurance companies and their Bermuda affiliates.

Risks Relating to Our Company

Our success will depend on our ability to maintain effective operating procedures and internal controls.

We continue to enhance our operating procedures and internal controls (including the timely and successful implementation of our information technology initiatives, which include the implementation of improved computerized systems and programs to replace and support manual systems, and including controls over financial reporting) to effectively support our business and our regulatory and reporting requirements. Our management does not expect that our disclosure controls or our internal controls will prevent all errors and all fraud. A control system, no matter how well conceived and operated, can provide only reasonable, not absolute, assurance that the objectives of the control system are met. Further, the design of a control system must reflect the fact that there are resource constraints, and the benefits of controls must be considered relative to their costs. As a result of the inherent limitations in

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all control systems, no evaluation of controls can provide absolute assurance that all control issues and instances of fraud, if any, within the company have been detected. These inherent limitations include the realities that judgments in decision-making can be faulty, and that breakdowns can occur because of simple error or mistake. Additionally, controls can be circumvented by the individual acts of some persons or by collusion of two or more people. The design of any system of controls also is based in part upon certain assumptions about the likelihood of future events, and there can be no assurance that any design will succeed in achieving its stated goals under all potential future conditions; over time, controls may become inadequate because of changes in conditions, or the degree of compliance with the policies or procedures may deteriorate. As a result of the inherent limitations in a cost-effective control system, misstatement due to error or fraud may occur and not be detected. Accordingly, our disclosure controls and procedures are designed to provide reasonable, not absolute, assurance that the disclosure controls and procedures are met.

The loss of our key employees or our inability to retain them could negatively impact our business.

Our success has been, and will continue to be, dependent on our ability to retain the services of our existing key executive officers and to attract and retain additional qualified personnel in the future. The pool of talent from which we actively recruit is limited. Although, to date, we have not experienced difficulties in attracting and retaining key personnel, the inability to attract and retain qualified personnel when available and the loss of services of key personnel could have a material adverse effect on our financial condition and results of operations. In addition, our underwriting staff is critical to our success in the production of business. While we do not consider any of our key executive officers or underwriters to be irreplaceable, the loss of the services of our key executive officers or underwriters or the inability to hire and retain other highly qualified personnel in the future could delay or prevent us from fully implementing our business strategy which could affect our financial performance. We are not aware of any intentions of any of our key personnel that would cause them no longer to provide their professional services to us in the near future.

The preparation of our financial statements requires us to make many estimates and judgments, which are even more difficult than those made in a mature company since limited historical information has been reported to us through September 30, 2005.

The preparation of consolidated financial statements requires us to make many estimates and judgments that affect the reported amounts of assets, liabilities (including reserves), revenues and expenses, and related disclosures of contingent liabilities. On an ongoing basis, we evaluate our estimates, including those related to revenue recognition, insurance and other reserves, reinsurance recoverables, investment valuations, intangible assets, bad debts, income taxes, contingencies and litigation. We base our estimates on historical experience, where possible, and on various other assumptions that we believe to be reasonable under the circumstances, which form the basis for our judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Estimates and judgments for a relatively new insurance and reinsurance company, like our company, are even more difficult to make than those made in a mature company since limited historical information has been reported to us through September 30, 2005. Instead, our current loss reserves are based almost entirely on estimates involving actuarial and statistical projections of our expectations of the ultimate settlement and administration costs of claims incurred but not yet reported. We utilize actuarial models as well as historical insurance industry loss development patterns to establish loss reserves. Accordingly, actual claims and claim expenses paid may deviate, perhaps substantially, from the reserve estimates reflected in our financial statements.

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The Warburg Pincus funds and the Hellman & Friedman funds together own a majority of our voting shares, and these shareholders have the right to have directors on our board; their interests may materially differ from the interests of the holders of our other securities.

The Warburg Pincus funds and the Hellman & Friedman funds own 31.5% and 19.7% of our outstanding voting shares, respectively, as of September 30, 2005. These shareholders are non-U.S. persons as defined in the Internal Revenue Code of 1986, as amended (the "Code"), and they are not subject to the voting limitation contained in our bye-laws. We have agreed not to declare any dividend or make any other distribution on our common shares, and not to repurchase any common shares, until we have repurchased from the Warburg Pincus funds and the Hellman & Friedman funds, pro rata, on the basis of the amount of these shareholders' investments in us at the time of such repurchase, preference shares (or common shares to which preference shares have been converted) having an aggregate value of $250 million, at a per share price acceptable to these shareholders. By reason of their ownership and the shareholders agreement between us and the Warburg Pincus funds and the Hellman & Friedman funds, the Warburg Pincus funds and the Hellman & Friedman funds, individually or together, are able to strongly influence or effectively control actions to be taken by us, or our shareholders.

In addition, the Warburg Pincus funds and the Hellman & Friedman funds are entitled to nominate a prescribed number of directors based on the respective retained percentages of their equity securities purchased in November 2001. Currently, our board consists of eleven members, which includes three directors nominated by the Warburg Pincus funds and two directors nominated by the Hellman & Friedman funds. As long as the Warburg Pincus funds retain at least 75% of their original investment and the Hellman & Friedman funds retain at least 60% of their original investment, these shareholders will be entitled to nominate six and three directors, respectively. Together they have the right to nominate a majority of directors to our board. The interests of these shareholders may differ materially from the interests of the holders of our other securities, and these shareholders could take actions or make decisions that are not in the interests of the holders of our other securities generally.

Our business is dependent upon insurance and reinsurance brokers, and the failure to develop or the loss of important broker relationships could materially adversely affect our ability to market our products and services.

We market our insurance and reinsurance products primarily through brokers. We derive a significant portion of our business from a limited number of brokers. During 2004, approximately 21.4% and 16.0% of our gross premiums written were generated from or placed by Marsh & McLennan Companies and its subsidiaries and AON Corporation and its subsidiaries, respectively. No other broker accounted for more than 10% of gross premiums written for the year ended December 31, 2004. Some of our competitors have had longer term relationships with the brokers we use than we have, and the brokers may promote products offered by companies that may offer a larger variety of products than we do. Loss of all or a substantial portion of the business provided by these brokers could have a material adverse effect on us.

We could be materially adversely affected to the extent that managing general agents, general agents and other producers in our program business exceed their underwriting authorities or otherwise breach obligations owed to us.

In the program business conducted by our insurance group, following our underwriting, financial, claims and information technology due diligence reviews, we authorize managing general agents, general agents and other producers to write business on our behalf within underwriting authorities prescribed by us. Once a program incepts, we must rely on the underwriting controls of these agents to write business within the underwriting authorities provided by us. Although we monitor our programs on an ongoing basis, our monitoring efforts may not be adequate or our agents may exceed their

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underwriting authorities or otherwise breach obligations owed to us. We have experienced breaches by certain of our agents, all of which have been resolved favorably for us. To the extent that our agents exceed their authorities or otherwise breach obligations owed to us in the future, our financial condition and results of operations could be materially adversely affected.

A downgrade in our ratings or our inability to obtain a rating for our operating insurance and reinsurance subsidiaries may adversely affect our relationships with clients and brokers and negatively impact sales of our products.

Financial strength and claims paying ratings from third party rating agencies are instrumental in establishing the competitive positions of companies in our industry. Periodically, rating agencies evaluate us to confirm that we continue to meet their criteria for the ratings assigned to us by them. Recently, rating agencies have been coming under increasing pressure as a result of high-profile corporate bankruptcies and may, as a result, increase their scrutiny of rated companies, revise their rating policies or take other action. Although, since the commencement of our underwriting initiative in October 2001, our ratings have not been downgraded, we can offer no assurances that our ratings will remain at their current levels. A ratings downgrade or the potential for such a downgrade or failure to obtain a necessary rating, could adversely affect both our relationships with agents, brokers, wholesalers and other distributors of our existing products and services and new sales of our products and services. Any ratings downgrade or failure to obtain a necessary rating could adversely affect our ability to compete in our markets and have a material adverse impact on our financial condition and results of operations.

Our investment performance may affect our financial results and ability to conduct business.

Our operating results depend in part on the performance of our investment portfolio. A significant portion of our cash and invested assets consists of fixed income securities (90.2% as of September 30, 2005). Although our current investment guidelines and approach stress preservation of capital, market liquidity and diversification of risk, our investments are subject to market-wide risks and fluctuations. In addition, although we did not experience any significant defaults by issuers during 2005, we are subject to risks inherent in particular securities or types of securities, as well as sector concentrations. We may not be able to realize our investment objectives, which could reduce our net income significantly. In the event that we are unsuccessful in correlating our investment portfolio with our expected insurance and reinsurance liabilities, we may be forced to liquidate our investments at times and prices that are not optimal, which could have a material adverse effect on our financial results and ability to conduct our business.

We may be adversely affected by interest rate changes.

Our operating results are affected, in part, by the performance of our investment portfolio. Our investment portfolio contains interest rate-sensitive-instruments, such as bonds, which may be adversely affected by changes in interest rates. Changes in interest rates could also have an adverse effect on our investment income and results of operations. For example, if interest rates increase, the value of our investment portfolio may decline.

In addition, our investment portfolio includes mortgage-backed securities. As of September 30, 2005, mortgage-backed securities (excluding commercial mortgage backed securities) constituted approximately 3.6% of our cash and invested assets. As with other fixed income investments, the fair market value of these securities fluctuates depending on market and other general economic conditions and the interest rate environment. Changes in interest rates can expose us to prepayment risks on these investments. In periods of declining interest rates, mortgage prepayments generally increase and mortgage-backed securities are prepaid more quickly, requiring us to reinvest the proceeds at the then current market rates.

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Interest rates are highly sensitive to many factors, including governmental monetary policies, domestic and international economic and political conditions and other factors beyond our control. Although we attempt to take measures to manage the risks of investing in a changing interest rate environment, we may not be able to mitigate interest rate sensitivity effectively. Despite our mitigation efforts, an increase in interest rates could have a material adverse effect on our book value.

We may require additional capital in the future, which may not be available or only available on unfavorable terms.

We monitor our capital adequacy on a regular basis. The capital requirements of our business depend on many factors, including our ability to write new business successfully and to establish premium rates and reserves at levels sufficient to cover losses. Our ability to underwrite is largely dependent upon the quality of our claims paying and financial strength ratings as evaluated by independent rating agencies. To the extent that our existing capital is insufficient to fund our future operating requirements and/or cover claim losses, we may need to raise additional funds through financings or limit our growth. Any equity or debt financing, if available at all, may be on terms that are unfavorable to us. In the case of equity financings, dilution to our shareholders could result, and, in any case, such securities may have rights, preferences and privileges that are senior to those of our outstanding securities. If we are not able to obtain adequate capital, our business, results of operations and financial condition could be adversely affected. See "Management's Discussion and Analysis of Financial Condition and Results of Operations—Liquidity and Capital Resources" in our Annual Report on Form 10-K for the year ended December 31, 2004 and in our Quarterly Report on Form 10-Q for the quarter ended September 30, 2005, each incorporated herein by reference.

We sold our prior reinsurance operations in May 2000 and may have liability to the purchaser and continuing liability from those reinsurance operations if the purchaser should fail to make payments on the reinsurance liabilities it assumed.

On May 5, 2000, we sold our prior reinsurance operations to Folksamerica. The Folksamerica transaction was structured as a transfer and assumption agreement (and not reinsurance), and, accordingly, the loss reserves (and any related reinsurance recoverables) relating to the transferred business are not included as assets or liabilities on our balance sheet. In addition, in connection with that asset sale, we made extensive representations and warranties about us and our reinsurance operations, some of which survived the closing of the asset sale. Breach of these representations and warranties could result in liability to us. In the event that Folksamerica refuses or is unable to make payment for reserved losses transferred to it by us in the May 2000 sale and the notice given to reinsureds is found not to be an effective release by such reinsureds, we would be liable for such claims.

We sold our non-standard automobile insurance operations and merchant banking operations in 2004 and may have liability to the purchasers.

In 2004, we sold our non-standard automobile insurance operations and merchant banking operations to third party purchasers. In connection with such sales, we made representations and warranties about us and our transferred businesses, some of which survived the closing of such sales. Breach of these representations and warranties could result in liability to us.

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Any future acquisitions may expose us to operational risks.

We have made, and may in the future make, strategic acquisitions, either of other companies or selected blocks of business. Any future acquisitions may expose us to operational challenges and risks, including:

•: integrating financial and operational reporting systems;
•: establishing satisfactory budgetary and other financial controls;
•: funding increased capital needs and overhead expenses;
•: obtaining management personnel required for expanded operations;
•: funding cash flow shortages that may occur if anticipated sales and revenues are not realized or are delayed, whether by general economic or market conditions or unforeseen internal difficulties;
•: the value of assets acquired may be lower than expected or may diminish due to credit defaults or changes in interest rates and liabilities assumed may be greater than expected;
•: the assets and liabilities we may acquire may be subject to foreign currency exchange rate fluctuation; and
•: financial exposures in the event that the sellers of the entities we acquire are unable or unwilling to meet their indemnification, reinsurance and other obligations to us.

Our failure to manage successfully these operational challenges and risks may impact our results of operations.

Some of the provisions of our bye-laws and our shareholders agreement may have the effect of hindering, delaying or preventing third party takeovers or changes in management initiated by shareholders. These provisions may also prevent our shareholders from receiving premium prices for their shares in an unsolicited takeover.

Some provisions of our bye-laws could have the effect of discouraging unsolicited takeover bids from third parties or changes in management initiated by shareholders. These provisions may encourage companies interested in acquiring the Company to negotiate in advance with our board of directors, since the board has the authority to overrule the operation of several of the limitations.

Among other things, our bye-laws provide:

•: for a classified board of directors, in which the directors of the class elected at each annual general meeting holds office for a term of three years, with the term of each class expiring at successive annual general meetings of shareholders;
•: that the number of directors is determined by the board from time to time by a vote of the majority of our board;
•: that directors may only be removed for cause, and cause removal shall be deemed to exist only if the director whose removal is proposed has been convicted of a felony or been found by a court to be liable for gross negligence or misconduct in the performance of his or her duties;
•: that our board has the right to fill vacancies, including vacancies created by an expansion of the board;
•: for limitations on shareholders' right to call special general meetings and to raise proposals or nominate directors at general meetings; and

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•: that shareholders may act by written consent only if such consent is unanimous among all shareholders entitled to vote.

Our bye-laws provide that certain provisions which may have anti-takeover effects may be repealed or altered only with prior board approval and upon the affirmative vote of holders of shares representing at least 65% of the total voting power of our shares entitled generally to vote at an election of directors.

The bye-laws also contain a provision limiting the rights of any U.S. person (as defined in Section 7701 (a) (30) of the Code) that owns shares of ACGL, directly, indirectly or constructively (within the meaning of Section 958 of the Code), representing more than 9.9% of the voting power of all shares entitled to vote generally at an election of directors. The votes conferred by such shares of such U.S. person will be reduced by whatever amount is necessary so that after any such reduction the votes conferred by the shares of such person will constitute 9.9% of the total voting power of all shares entitled to vote generally at an election of directors. Notwithstanding this provision, the board may make such final adjustments to the aggregate number of votes conferred by the shares of any U.S. person that the board considers fair and reasonable in all circumstances to ensure that such votes represent 9.9% of the aggregate voting power of the votes conferred by all shares of ACGL entitled to vote generally at an election of directors. ACGL will assume that all shareholders (other than the Warburg Pincus funds and the Hellman & Friedman funds) are U.S. persons unless we receive assurance satisfactory to us that they are not U.S. persons.

Moreover, most states, including states in which our subsidiaries are domiciled, have laws and regulations that require regulatory approval of a change in control of an insurer or an insurer's holding company. Where such laws apply to us and our subsidiaries, there can be no effective change in our control unless the person seeking to acquire control has filed a statement with the regulators and has obtained prior approval for the proposed change from such regulators. The usual measure for a presumptive change in control pursuant to these laws is the acquisition of 10% or more of the voting power of the insurance company or its parent, although this presumption is rebuttable. Consequently, a person may not acquire 10% or more of our common shares without the prior approval of insurance regulators in the state in which our subsidiaries are domiciled.

The bye-laws also provide that the affirmative vote of 80% of our outstanding shares (including a majority of the outstanding shares held by shareholders other than holders (and such holders' affiliates) of 10% or more ("10% holders") of the outstanding shares) shall be required (the "extraordinary vote") for the following corporate actions:

•: merger or consolidation of the Company into a 10% holder;
•: sale of any or all of our assets to a 10% holder;
•: the issuance of voting securities to a 10% holder; or
•: amendment of these provisions;

provided, however, the extraordinary vote will not apply to any transaction approved by the board, so long as a majority of those board members voting in favor of the transaction were duly elected and acting members of the board prior to the time the 10% holder became a 10% holder.

The provisions described above may have the effect of making more difficult or discouraging unsolicited takeover bids from third parties. To the extent that these effects occur, shareholders could be deprived of opportunities to realize takeover premiums for their shares and the market price of their shares could be depressed. In addition, these provisions could also result in the entrenchment of incumbent management.

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Our operating insurance and reinsurance subsidiaries are subject to regulation in various jurisdictions, and material changes in the regulation of their operations could adversely affect our results of operations.

Our insurance and reinsurance subsidiaries are subject to government regulation in each of the jurisdictions in which they are licensed or authorized to do business. Governmental agencies have broad administrative power to regulate many aspects of the insurance business, which may include trade and claim practices, accounting methods, premium rates, marketing practices, claims practices, advertising, policy forms, and capital adequacy. These agencies are concerned primarily with the protection of policyholders rather than shareholders. Moreover, insurance laws and regulations, among other things:

•: establish solvency requirements, including minimum reserves and capital and surplus requirements;
•: limit the amount of dividends, tax distributions, intercompany loans and other payments our insurance subsidiaries can make without prior regulatory approval;
•: impose restrictions on the amount and type of investments we may hold; and
•: require assessments to pay claims of insolvent insurance companies.

The National Association of Insurance Commissioners, which we call the NAIC, continuously examines existing laws and regulations in the United States. We cannot predict the effect that any NAIC recommendations or proposed or future legislation or rule making in the United States or elsewhere may have on our financial condition or operations.

Our Bermuda insurance and reinsurance subsidiary, Arch Re Bermuda, conducts its business from its offices in Bermuda and is not licensed or admitted to do business in any jurisdiction except Bermuda. We do not believe that Arch Re Bermuda is subject to the insurance laws of any state in the United States; however, recent scrutiny of the insurance and reinsurance industry in the U.S. and other countries could subject Arch Re Bermuda to additional regulation. Our U.S. reinsurance subsidiary, Arch Re U.S., and our U.S. insurance subsidiaries, Arch Insurance, Arch Specialty, Arch E&S and Western Diversified, write reinsurance and insurance in the United States. These subsidiaries are subject to extensive regulation under state statutes which delegate regulatory, supervisory and administrative powers to state insurance commissioners. Such regulation generally is designed to protect policyholders rather than investors.

Arch-Europe conducts its business from its offices in London and Germany and is regulated by the UK Financial Services Authority. Arch-Europe is licensed to write insurance business in the U.K. and European Union countries and is subject to insurance laws in the U.K. and the European Union. Arch-Europe also has branch licenses to write insurance in Germany, Italy and Spain. In addition, the Canadian branch of Arch Insurance received its federal license to underwrite insurance in Canada in January 2005.

Each of our U.S., Bermuda and U.K. insurance and reinsurance subsidiaries is required to maintain minimum capital and surplus as mandated by their respective jurisdictions of incorporation and, in some cases, by the jurisdictions in which those subsidiaries write business. All of our subsidiaries are currently in compliance with these capital and surplus requirements.

We periodically review our corporate structure in the United States so that we can optimally deploy our capital. Changes in that structure require regulatory approval. Delays or failure in obtaining any of these approvals could limit the amount of insurance that we can write in the United States.

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imposition of significant restrictions on our ability to do business, and could also result in fines and other sanctions, any or all of which could adversely affect our financial condition and results of operations.

ACGL is a holding company and is dependent on dividends and other payments from its operating subsidiaries, which are subject to dividend restrictions, to make payments, including any payments of dividends, redemption amounts or liquidation amounts on the Series A Non-Cumulative Preferred Shares.

ACGL is a holding company whose assets primarily consist of the shares in our subsidiaries. Generally, we depend on our available cash resources, liquid investments and dividends or other distributions from our subsidiaries to make payments, including the payment of debt service obligations and operating expenses we may incur and any dividends or liquidation amounts with respect to the Series A Non-Cumulative Preferred Shares. The ability of our regulated insurance and reinsurance subsidiaries to pay dividends or make distributions is dependent on their ability to meet applicable regulatory standards. Since the commencement of our underwriting initiative in October 2001 through December 31, 2005, no dividends were paid to ACGL by any of its subsidiaries, other than dividends of $11.0 million paid by Arch Re Bermuda to ACGL in October 2005 and 2004, which were used to pay interest on ACGL's senior notes. We believe that ACGL has enough cash resources and available dividend capacity to service its indebtedness and other current outstanding obligations.

The ability of our regulated insurance and reinsurance subsidiaries to pay dividends or make distributions is dependent on their ability to meet applicable regulatory standards. Under Bermuda law, Arch Re Bermuda is required to maintain a minimum solvency margin (i.e., the amount by which the value of its general business assets must exceed its general business liabilities) equal to the greatest of (1) $100 million, (2) 50% of net premiums written (being gross premiums written by us less any premiums ceded by us, but we may not deduct more than 25% of gross premiums when computing net premiums written) and (3) 15% of loss and other insurance reserves. Arch Re Bermuda is prohibited from declaring or paying any dividends during any financial year if it is not in compliance with its minimum solvency margin. In addition, Arch Re Bermuda is prohibited from declaring or paying in any financial year dividends of more than 25% of its total statutory capital and surplus (as shown on its previous financial year's statutory balance sheet) unless it files, at least seven days before payment of such dividends, with the Bermuda Monetary Authority an affidavit stating that it will continue to meet the required margins. In addition, Arch Re Bermuda is prohibited, without prior approval of the Bermuda Monetary Authority, from reducing by 15% or more its total statutory capital, as set out in its previous year's statutory financial statements. At December 31, 2004, as determined under Bermuda law, Arch Re Bermuda had statutory capital of $1.57 billion and statutory capital and surplus of $1.94 billion. Such amounts include interests in U.S. insurance and reinsurance subsidiaries. Accordingly, approximately $235 million was available for dividends or distributions during 2005 without prior approval under Bermuda law, as discussed above. Our U.S. insurance and reinsurance subsidiaries were able to pay $29.8 million in dividends or distributions to Arch Re Bermuda during 2005 without prior regulatory approval, but such dividends or distributions would have been subject to applicable withholding or other taxes.

In addition, the ability of our insurance and reinsurance subsidiaries to pay dividends could be constrained by our dependence on financial strength ratings from independent rating agencies. Our ratings from these agencies depend to a large extent on the capitalization levels of our insurance and reinsurance subsidiaries.

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If our Bermuda reinsurance subsidiary is unable to provide collateral to ceding companies, its ability to conduct business could be significantly and negatively affected.

Arch Re Bermuda is a registered Bermuda insurance company and is not licensed or admitted as an insurer in any jurisdiction in the United States. Because insurance regulations in the United States do not permit insurance companies to take credit for reinsurance obtained from unlicensed or non-admitted insurers on their statutory financial statements unless security is posted, Arch Re Bermuda's contracts generally require it to post a letter of credit or provide other security. Although, to date, Arch Re Bermuda has not experienced any difficulties in providing collateral when required, if we are unable to post security in the form of letters of credit or trust funds when required, the operations of Arch Re Bermuda could be significantly and negatively affected.

We may become subject to taxes in Bermuda after March 28, 2016, which may have a material adverse effect on our results of operations.

Under current Bermuda law, we are not subject to tax on income or capital gains. Furthermore, we have obtained from the Minister of Finance of Bermuda under the Exempted Undertakings Tax Protection Act, 1966, an assurance that, in the event that Bermuda enacts legislation imposing tax computed on profits, income, any capital asset, gain or appreciation, or any tax in the nature of estate duty or inheritance tax, then the imposition of the tax will not be applicable to us or our operations until March 28, 2016. We could be subject to taxes in Bermuda after that date. This assurance does not, however, prevent the imposition of taxes on any person ordinarily resident in Bermuda or any company in respect of its ownership of real property or leasehold interests in Bermuda.

Foreign currency exchange rate fluctuation may adversely affect our financial results.

We write business on a worldwide basis, and our results of operations may be affected by fluctuations in the value of currencies other than the U.S. Dollar. The primary foreign currencies in which we operate are the Euro, the British Pound Sterling and the Canadian Dollar. Changes in foreign currency exchange rates can reduce our revenues and increase our liabilities and costs. We may therefore suffer losses solely as a result of exchange rate fluctuations. In order to mitigate the impact of exchange rate fluctuations, we have invested and expect to continue to invest in securities denominated in currencies other than the U.S. Dollar. Net foreign exchange losses, recorded in the statement of income, for the year ended December 31, 2004 were $17.4 million, compared to net foreign exchange gains for the year ended December 31, 2003 of $1.0 million. Our reinsurance segment invests certain funds in British Pounds Sterling and Euros in order to mitigate the economic effects of foreign currency exchange risk on projected liabilities in such currencies. Such investments appreciated, due to changes in foreign currency rates, during the year ended December 31, 2004 by $15.4 million. However, such gains are reflected as a direct increase to shareholders' equity and are not included in the statement of income. We have chosen not to hedge the currency risk on the capital contributed to Arch-Europe in May 2004, which is held in British Pounds Sterling. However, we intend to match Arch-Europe's projected liabilities in foreign currencies with investments in the same currencies. There can be no assurances that such arrangements will mitigate the negative impact of exchange rate fluctuations, and we may suffer losses solely as a result of exchange rate fluctuations. Since inception, and based on currency spot rates at December 31, 2004, Arch Re Bermuda has recorded net premiums written of approximately $217.2 million from British Pound Sterling-denominated contracts, $335.2 million from Euro-denominated contracts and $135.2 million from Canadian Dollar-denominated contracts.

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Employees of our Bermuda operations are required to obtain work permits before engaging in a gainful occupation in Bermuda. Required work permits may not be granted or may not remain in effect.

Under Bermuda law, only persons who are Bermudians, spouses of Bermudians, holders of a permanent resident's certificate or holders of a working resident's certificate ("exempted persons") may engage in gainful occupation in Bermuda without an appropriate governmental work permit. Our success may depend in part on the continued services of key employees in Bermuda. A work permit may be granted or renewed upon showing that, after proper public advertisement, no exempted person is available who meets the minimum standards reasonably required by the employer. The Bermuda government's policy places a six-year term limit on individuals with work permits, subject to certain exemptions for key employees. A work permit is issued with an expiry date (up to five years) and no assurances can be given that any work permit will be issued or, if issued, renewed upon the expiration of the relevant term. We consider our key officers in Bermuda to be Constantine Iordanou, our President and Chief Executive Officer (work permit expires November 12, 2006), Marc Grandisson, Chairman and Chief Executive Officer of Arch Worldwide Re-Insurance Group (work permit expires May 12, 2010), John D. Vollaro, our Executive Vice President and Chief Financial Officer (work permit expires July 25, 2010) and Nicolas Papadopoulo, President and Chief Executive Officer of Arch Re Bermuda (work permit expires March 31, 2010). We also have other key positions in Bermuda held by persons who hold work permits subject to renewal. If work permits are not obtained or renewed for our principal employees, we could lose their services, which could materially affect our business.

The enforcement of civil liabilities against us may be difficult.

We are a Bermuda company and in the future some of our officers and directors may be residents of various jurisdictions outside the United States. All or a substantial portion of our assets and of those persons may be located outside the United States. As a result, it may be difficult for you to effect service of process within the United States upon those persons or to enforce in United States courts judgments obtained against those persons.

We have appointed National Registered Agents, Inc., New York, New York, as our agent for service of process with respect to actions based on offers and sales of securities made in the United States. We have been advised by our Bermuda counsel, Conyers Dill & Pearman, that the United States and Bermuda do not currently have a treaty providing for reciprocal recognition and enforcement of judgments of U.S. courts in civil and commercial matters and that a final judgment for the payment of money rendered by a court in the United States based on civil liability, whether or not predicated solely upon the U.S. federal securities laws, would, therefore, not be automatically enforceable in Bermuda. We also have been advised by Conyers Dill & Pearman that a final and conclusive judgment obtained in a court in the United States under which a sum of money is payable as compensatory damages (i.e., not being a sum claimed by a revenue authority for taxes or other charges of a similar nature by a governmental authority, or in respect of a fine or penalty or multiple or punitive damages) may be the subject of an action on a debt in the Supreme Court of Bermuda under the common law doctrine of obligation. Such an action should be successful upon proof that the sum of money is due and payable, and without having to prove the facts supporting the underlying judgment, as long as:

•: the court which gave the judgment had proper jurisdiction over the parties to such judgment;
•: such court did not contravene the rules of natural justice of Bermuda;
•: such judgment was not obtained by fraud;
•: the enforcement of the judgment would not be contrary to the public policy of Bermuda;
•: no new admissible evidence relevant to the action is submitted prior to the rendering of the judgment by the courts of Bermuda; and
•: there is due compliance with the correct procedures under Bermuda law.

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A Bermuda court may impose civil liability on us or our directors or officers in a suit brought in the Supreme Court of Bermuda against us or such persons with respect to a violation of U.S. federal securities laws, provided that the facts surrounding such violation would constitute or give rise to a cause of action under Bermuda law.

Risk Factors Relating to the Series A Non-Cumulative Preferred Shares

General market conditions and unpredictable factors could adversely affect market prices for the Series A Non-Cumulative Preferred Shares.

There can be no assurance about the market prices for the Series A Non-Cumulative Preferred Shares. Several factors, many of which are beyond our control, will influence the market value of the Series A Non-Cumulative Preferred Shares. Factors that might influence the market value of the Series A Non-Cumulative Preferred Shares include, but are not limited to:

•: whether dividends have been declared and are likely to be declared on the Series A Non-Cumulative Preferred Shares from time to time;
•: our creditworthiness, financial condition, performance and prospects;
•: whether the ratings on the Series A Non-Cumulative Preferred Shares provided by any ratings agency have changed;
•: the market for similar securities; and
•: economic, financial, geopolitical, regulatory or judicial events that affect us and/or the insurance or financial markets generally.

If you purchase Series A Non-Cumulative Preferred Shares, the Series A Non-Cumulative Preferred Shares may subsequently trade at a discount to the price that you paid for them.

Dividends on the Series A Non-Cumulative Preferred Shares are non-cumulative.

Dividends on the Series A Non-Cumulative Preferred Shares are non-cumulative and payable only out of lawfully available funds under Bermuda law. Consequently, if the board of directors of Arch Capital (or a duly authorized committee of the board) does not authorize and declare a dividend for any dividend period, holders of the Series A Non-Cumulative Preferred Shares would not be entitled to receive any such dividend, and such unpaid dividend will not accrue and will never be payable. We will have no obligation to pay dividends for a dividend period on or after the dividend payment date for such period if the board of directors of Arch Capital (or a duly authorized committee of the board) has not declared such dividend before the related dividend payment date; if dividends on the Series A Non-Cumulative Preferred Shares are authorized and declared with respect to any subsequent dividend period, we will be free to pay dividends on any other series of preferred shares and/or our common shares. In the past, we have not paid dividends on our common shares.

The Series A Non-Cumulative Preferred Shares are equity and are subordinate to our existing and future indebtedness.

The Series A Non-Cumulative Preferred Shares are equity interests and do not constitute indebtedness. As such, the Series A Non-Cumulative Preferred Shares will rank junior to all of our indebtedness and other non-equity claims with respect to assets available to satisfy our claims, including in our liquidation. As of September 30, 2005, our total consolidated long-term debt was $300.0 million. We may incur additional debt in the future. In November 2005, we entered into a new revolving credit and letter of credit facility. See "Prospectus Supplement Summary—Recent Developments" in this prospectus supplement. Our existing and future indebtedness may restrict payments of dividends on the Series A Non-Cumulative Preferred Shares. Additionally, unlike indebtedness, where principal and interest would customarily be payable on specified due dates, in the case of preferred shares like the Series A Non-Cumulative Preferred Shares (1) dividends are payable only if declared by the board of directors of Arch Capital (or a duly authorized committee of the board) and (2) as described above

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under "—Risks Relating to our Company—AGCL is a holding company and is dependent on dividends and other payments from its operating subsidiaries, which are subject to dividend restrictions, to make payments, including any payments of dividends or liquidation amounts on the Series A Non-Cumulative Preferred Shares," we are subject to certain regulatory and other constraints affecting our ability to pay dividends and make other payments.

You may be unable to sell your Series A Non-Cumulative Preferred Shares if an active trading market does not develop.

The Series A Non-Cumulative Preferred Shares are a new issue with no established trading market. Although we intend to apply to have the Series A Non-Cumulative Preferred Shares approved for listing on the NYSE, there may be little or no secondary market for the Series A Non-Cumulative Preferred Shares. Even if a secondary market for the Series A Non-Cumulative Preferred Shares develops, it may not provide significant liquidity, and transaction costs in any secondary market could be high. As a result, the difference between bid and ask prices in any secondary market could be substantial.

The voting rights of holders of the Series A Non-Cumulative Preferred Shares will be limited.

Holders of the Series A Non-Cumulative Preferred Shares have no voting rights with respect to matters that generally require the approval of voting shareholders. The limited voting rights of holders of the Series A Non-Cumulative Preferred Shares include the right to vote as a class on certain fundamental matters that affect the preference or special rights of the Series A Non-Cumulative Preferred Shares, as described under "Description of the Series A Non-Cumulative Preferred Shares—Voting Rights" in this prospectus supplement. In addition, if dividends on the Series A Non-Cumulative Preferred Shares have not been declared or paid for the equivalent of six dividend payments, whether or not for consecutive dividend periods, holders of the outstanding Series A Non-Cumulative Preferred Shares, together with holders of any other series of our preferred shares ranking equal with the Series A Non-Cumulative Preferred Shares with similar voting rights, will be entitled to vote for the election of two additional directors to our board of directors subject to the terms and to the limited extent described under "Description of the Series A Non-Cumulative Preferred Shares—Voting Rights" in this prospectus supplement.

There is no limitation on our issuance of securities that rank equally with or senior to the Series A Non-Cumulative Preferred Shares.

We may issue securities that rank equally with or senior to the Series A Non-Cumulative Preferred Shares without limitation. The issuance of securities ranking equally with or senior to the Series A Non-Cumulative Preferred Shares may reduce the amount available for dividends and the amount recoverable by holders of the Series A Non-Cumulative Preferred Shares in the event of a liquidation, dissolution or winding-up of Arch Capital.

Risks Relating to Taxation

We and our non-U.S. subsidiaries may become subject to U.S. federal income taxation.

ACGL and its non-U.S. subsidiaries intend to operate their business in a manner that will not cause them to be treated as engaged in a trade or business in the United States and, thus, will not be required to pay U.S. federal income taxes (other than U.S. excise taxes on insurance and reinsurance premium and withholding taxes on certain U.S. source investment income) on their income. However, because there is uncertainty as to the activities which constitute being engaged in a trade or business in the United States, there can be no assurances that the U.S. Internal Revenue Service will not contend successfully that ACGL or its non-U.S. subsidiaries are engaged in a trade or business in the United States. If ACGL or any of its non-U.S. subsidiaries were subject to U.S. income tax, our shareholders' equity and earnings could be adversely affected. Certain of our U.S. subsidiaries have been personal holding companies, but did not have "undistributed personal holding company income."

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Congress has been considering legislation intended to eliminate certain perceived tax advantages of Bermuda insurance companies and U.S. insurance companies having Bermuda affiliates, including perceived tax benefits resulting principally from reinsurance between or among U.S. insurance companies and their Bermuda affiliates. In this regard, the American Jobs Creation Act of 2004 (the "Jobs Act") permits the United States Internal Revenue Service ("IRS") to re-allocate, re-characterize or adjust items of income, deduction or certain other items related to a reinsurance agreement between related parties to reflect the proper source, character and amount for each item (in contrast to prior law, which only covered source and character). The Jobs Act also eliminated the tax benefits available to a U.S. company that, after a certain date, changed its legal domicile to a non-U.S. jurisdiction, a transaction commonly known as an inversion. We changed our legal domicile from the U.S. to Bermuda, but were not affected by the anti-inversion rule because our change in domicile occurred in November 2000. Additional legislation, if passed, and other changes in U.S. tax laws, regulations and interpretations thereof to address these issues could adversely affect us.

U.S. persons who hold our Series A Non-Cumulative Preferred Shares may be subject to U.S. income taxation at ordinary income rates on our undistributed earnings and profits.

We believe that we and our non-U.S. subsidiaries currently are controlled foreign corporations ("CFCs"), although our bye-laws are designed to preclude any U.S. person from adverse tax consequences as a result of our CFC status. We do not believe that we are a passive foreign investment company or "PFIC". As these determinations and beliefs are based upon legal and factual conclusions, no assurances can be given that the U.S. Internal Revenue Service or a court would concur with our conclusions. If they were not to so concur, U.S. persons who hold our or our Series A Non-Cumulative Preferred Shares may suffer adverse tax consequences.

Reduced tax rate for qualified dividend income received by individuals and other non-corporate holders may not be available in the future.

If, as is intended, the Series A Non-Cumulative Preferred Shares are approved for listing on the NYSE, dividends received by individuals and other non-corporate United States Persons on the Series A Non-Cumulative Preferred Shares on or before December 31, 2008 should constitute qualified dividend income that is subject to U.S. federal income tax at the rate applicable for long-term capital gains, rather than the higher rates applicable to ordinary income, provided that certain holding period requirements and other conditions are met. There is a risk that dividends, if any, paid prior to the listing of the Series A Non-Cumulative Preferred Shares on the NYSE may not constitute qualified dividend income. See "Certain Income Tax Considerations—Taxation of Shareholders—United States Taxation—Taxation of Dividends." After December 31, 2008, qualified dividend income will no longer be taxed at the rate applicable for long-term capital gains unless legislation is enacted providing otherwise. A legislative proposal passed by the Ways and Means Committee in the House of Representatives would extend the preferential treatment of qualified dividend income to dividends paid prior to December 31, 2010, but this proposal has not been included in the Senate-passed Tax Reconciliation Bill, and no assurances can be given that the proposal will be enacted. A legislative proposal introduced in the Senate would increase the holding period requirement for dividends paid on preferred stock. While this proposal has not been included in the Senate-passed Tax Reconciliation Bill, no assurances can be given that this or a similar proposal will not be enacted.

Our non-U.S. companies may be subject to U.K. tax that may have a material adverse effect on our results of operations.

We intend to operate in such a manner so that none of our companies, other than Arch-Europe, should be resident in the U.K. for tax purposes or have a permanent establishment in the U.K. Accordingly, we expect that none of our companies other than Arch-Europe should be subject to U.K. taxation. However, since applicable law and regulations do not conclusively define the activities that constitute conducting business in the U.K. through a permanent establishment, the U.K. Inland Revenue might contend successfully that one or more of our companies, in addition to Arch-Europe, is conducting business in the U.K. through a permanent establishment in the U.K. and, therefore, subject to U.K. tax, which could have a material adverse effect on us.

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USE OF PROCEEDS

We expect to receive approximately $192 million in net proceeds from the sale of Series A Non-Cumulative Preferred Shares issued in this offering. We intend to use the net proceeds of this offering for general corporate purposes.

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RATIO OF EARNINGS TO FIXED CHARGES AND PREFERENCE SHARE DIVIDENDS

The ratio of earnings to fixed charges for each of the periods set forth below is as follows:

		Year Ended December 31,										Nine Months Ended September 30,
		2004		2003		2002		2001		2000		2005
Ratio of earnings to fixed charges(1)(2)		16.5x		71.4x		50.1x		488.5x		N/Ax(3)		9.6x

(1): For purposes of determining the ratio of earnings to fixed charges, "earnings" consists of (a) income before income taxes, extraordinary item and cumulative effect of accounting change, minus (b) equity in net income of investees, plus (c) fixed charges, and "fixed charges" consists of (a) interest and amortization on indebtedness, plus (b) estimate of interest component within rental expense net of sublease income.
(2): Our series A convertible preference shares, the remaining outstanding of which were converted to common shares in December 2005, were non-dividend bearing securities.
(3): For the year ended December 31, 2000, Arch Capital's income was insufficient by $1,379,000 to cover its fixed charges.

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CAPITALIZATION

The following table sets forth our capitalization at September 30, 2005 on:

•: an actual basis; and
•: an as adjusted basis to give effect to (a) the issuance of the Series A Non-Cumulative Preferred Shares in this offering and (b) the conversion of the remaining outstanding series A convertible preference shares into a like number of our common shares, which occurred in the fourth quarter of 2005 (see "Prospectus Supplement Summary—Recent Developments"), as if each had occurred on September 30, 2005.

The following should be read in conjunction with our financial statements and the notes related thereto which are included in our Quarterly Report on Form 10-Q for the fiscal quarter ended September 30, 2005 incorporated herein by reference.

		September 30, 2005
		Actual			As adjusted
		(in thousands)
7.35% Senior Notes due 2034		$	300,000		$	300,000
Revolving credit facility borrowings(1)			—			—

	Total debt(2)		300,000			300,000

Shareholders' equity:
Series A convertible preference shares, $0.01 par value, 50,000,000 shares authorized, 37,327,502 shares issued and outstanding, actual			373			—
Series A Non-Cumulative Preferred Shares offered hereby (8,000,000 issued and outstanding, as adjusted)			—			80
Common shares, $0.01 par value, 200,000,000 shares authorized, and 35,504,734 shares issued and outstanding, actual and 72,832,236 issued and outstanding, as adjusted(3)			355			728
Additional paid-in capital			1,575,843			1,768,463
Deferred compensation under share award plan			(5,625	)		(5,625	)
Retained earnings			800,455			800,455
Accumulated other comprehensive income consisting of appreciation in value of investments, net of deferred income tax			(19,870	)		(19,870	)

	Total shareholders' equity		2,351,531			2,544,231

	Total capitalization(4)	$	2,651,531		$	2,844,231

Selected ratios:
	Ratio of total debt to total capitalization		11.3	%		10.5	%
	Ratio of Series A Non-Cumulative Preferred Shares to total capitalization (as adjusted)		—			7.0	%

(1): In November 2005, we entered into new revolving credit and letter of credit facilities. See "Prospectus Supplement Summary—Recent Developments—Closing of new revolving loan and letter of credit facilities."
(2): Does not include reserves or other balance sheet or non-balance-sheet liabilities, including contingent liabilities. See the sections entitled "Selected Historical Consolidated Financial and Operating Data" and "Management's Discussion and Analysis of Financial Condition and Results of Operations in our Quarterly Report on Form 10-Q for the fiscal quarter ended September 30, 2005 incorporated herein by reference.
(3): The number of common shares outstanding excludes the effect of 5,859,027 outstanding employee stock options and 95,500 restricted stock units outstanding at September 30, 2005.
(4): Total capitalization is comprised of total debt and total shareholders' equity.

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DESCRIPTION OF THE SERIES A NON-CUMULATIVE PREFERRED SHARES

The following description of the particular terms of the Series A Non-Cumulative Preferred Shares supplements the description of the general terms and provisions of the preference shares set forth under "Description of Arch Capital Preference Shares" in the accompanying prospectus. The following summary of the terms and provisions of the Series A Non-Cumulative Preferred Shares does not purport to be complete and is qualified in its entirety by reference to the pertinent sections of the bye-laws of Arch Capital, which we have previously filed with the Securities and Exchange Commission, or SEC, and the Certificate of Designations creating the Series A Non-Cumulative Preferred Shares, which will be included as an exhibit to documents that we file with the SEC. Terms used in this prospectus supplement that are otherwise not defined will have the meanings given to them in the accompanying prospectus. As used in this section, "we," "us," "our," "the Company," "Arch Capital" and "ACGL" mean Arch Capital Group Ltd. and do not include its subsidiaries.

General

In January 2006, a duly authorized committee of our board of directors approved the Certificate of Designations setting forth the specific rights, preferences, limitations and other terms of the Series A Non-Cumulative Preferred Shares. The Series A Non-Cumulative Preferred Shares constitute a series of our authorized preference shares.

We will generally be able to pay dividends and distributions upon liquidation, dissolution or winding-up only out of lawfully available funds for such payment (i.e., after taking account of all indebtedness and other non-equity claims). The Series A Non-Cumulative Preferred Shares will be fully paid and non-assessable when issued, which means that holders will have paid their purchase price in full and that we may not ask them to surrender additional funds. Holders of the Series A Non-Cumulative Preferred Shares will not have preemptive or subscription rights to acquire more of our capital shares.

The Series A Non-Cumulative Preferred Shares will not be convertible into, or exchangeable for, shares of any other class or series of shares or other securities of ours. The Series A Non-Cumulative Preferred Shares have no stated maturity and will not be subject to any sinking fund, retirement fund or purchase fund or other obligation of Arch Capital to redeem or purchase the Series A Non-Cumulative Preferred Shares.

Ranking

The Series A Non-Cumulative Preferred Shares will rank senior to our junior shares (as defined herein) and equally with each other series of our preferred shares that we may issue with respect to the payment of dividends and distributions of assets upon liquidation, dissolution or winding-up. As of the date of this prospectus supplement, we have no outstanding series or class of shares that are on parity with the Series A Non-Cumulative Preferred Shares with respect to payment of dividends and the distribution of assets upon a liquidation, dissolution or winding-up of Arch Capital. Unless our shareholders otherwise provide, our board of directors may from time to time create and issue preference shares of other series and fix their relative rights, preferences and limitations.

Dividends

Dividends on the Series A Non-Cumulative Preferred Shares will not be mandatory. Holders of Series A Non-Cumulative Preferred Shares will be entitled to receive only when, as and if declared by the board of directors of Arch Capital or a duly authorized committee of the board, out of lawfully available funds for the payment of dividends under Bermuda law, non-cumulative cash dividends from the original issue date, quarterly on the 15^th day of February, May, August and November of each year, commencing on May 15, 2006. These dividends will accrue with respect to a particular dividend period, on the liquidation preference amount of $25 per share at an annual rate of 8.00%. In the event that we

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issue additional Series A Non-Cumulative Preferred Shares after the original issue date, dividends on such additional shares may accrue from the original issue date or any other date we specify at the time such additional shares are issued.

Dividends, if so declared, will be payable to holders of record of the Series A Non-Cumulative Preferred Shares as they appear on our books on the applicable record date, which shall be the 15th calendar day of the month immediately preceeding the applicable dividend payment date or such other record date fixed by our board of directors (or a duly authorized committee of the board) that is not more than 60 nor less than 10 days prior to such dividend payment date (each, a "dividend record date"). These dividend record dates will apply regardless of whether a particular dividend record date is a business day.

A dividend period is the period from and including a dividend payment date to but excluding the next dividend payment date, except that the initial dividend period will commence on and include the original issue date of the Series A Non-Cumulative Preferred Shares and will end on and exclude the May 15, 2006 dividend payment date. Dividends payable on the Series A Non-Cumulative Preferred Shares will be computed on the basis of a 360-day year consisting of twelve 30-day months. If any date on which dividends would otherwise be payable is not a business day, then the dividend payment date will be the next succeeding business day with the same force and effect as if made on the original dividend payment date, and no additional dividends shall accrue on the amount so payable from such date to such next succeeding business day.

Dividends on the Series A Non-Cumulative Preferred Shares will not be cumulative. Accordingly, if the board of directors of Arch Capital, or a duly authorized committee of the board, does not declare a dividend on the Series A Non-Cumulative Preferred Shares payable in respect of any dividend period before the related dividend payment date, such dividend will not accrue and will not be payable and we will have no obligation to pay a dividend for that dividend period on the dividend payment date or at any future time, whether or not dividends are declared for any future dividend period on the Series A Non-Cumulative Preferred Shares or any other preferred shares we may issue in the future.

So long as any Series A Non-Cumulative Preferred Shares remain outstanding for any dividend period, unless the full dividends for the latest completed dividend period on all outstanding Series A Non-Cumulative Preferred Shares and parity shares have been declared and paid (or declared and a sum sufficient for the payment thereof has been set aside):

•: no dividend shall be paid or declared on our common shares, or any other junior shares (as defined below) (other than a dividend payable solely in junior shares); and
•: no common shares or other junior shares shall be purchased, redeemed or otherwise acquired for consideration by us, directly or indirectly (other than (i) as a result of a reclassification of junior shares for or into other junior shares, or the exchange or conversion of one junior share for or into another junior share, (ii) through the use of the proceeds of a substantially contemporaneous sale of junior shares and (iii) as permitted by the bye-laws of Arch Capital in effect on the date of issuance of the Series A Non-Cumulative Preferred Shares).

As used in this prospectus supplement, "junior shares" means any class or series of our capital shares that ranks junior to the Series A Non-Cumulative Preferred Shares either as to the payment of dividends or as to the distribution of assets upon any liquidation, dissolution or winding-up of Arch Capital. At present, junior shares consist of our common shares.

When dividends are not paid (or duly provided for) in full on any dividend payment date (or, in the case of parity shares (as defined below) having dividend payment dates different from the dividend payment dates pertaining to the Series A Non-Cumulative Preferred Shares, on a dividend payment date falling within the related dividend period for the Series A Non-Cumulative Preferred Shares) upon the Series A Non-Cumulative Preferred Shares and any parity shares, all dividends declared by the board of directors of Arch Capital or a duly authorized committee of the board upon the Series A

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Non-Cumulative Preferred Shares and all such parity shares and payable on such dividend payment date (or, in the case of parity shares having dividend payment dates different from the dividend payment dates pertaining to the Series A Non-Cumulative Preferred Shares, on a dividend payment date falling within the related dividend period for the Series A Non-Cumulative Preferred Shares) shall be declared by the board or such committee pro rata so that the respective amounts of such dividends shall bear the same ratio to each other as all declared but unpaid dividends per Series A Non-Cumulative Preferred Share and all parity shares payable on such dividend payment date (or, in the case of parity shares having dividend payment dates different from the dividend payment dates pertaining to the Series A Non-Cumulative Preferred Shares, on a dividend payment date falling within the related dividend period for the Series A Non-Cumulative Preferred Shares) bear to each other.

As used in this prospectus supplement, "parity shares" means any class or series of our capital shares that ranks equally with the Series A Non-Cumulative Preferred Shares with respect to the payment of dividends and in the distribution of assets on any liquidation, dissolution or winding-up of Arch Capital. As of the date of this prospectus supplement, we have no outstanding class or series of shares that are parity shares.

Certain Bermuda Restrictions on Payment of Dividends

Under Bermuda law, we may not lawfully declare or pay a dividend if we have reasonable grounds for believing that we are, and would after payment of the dividend be, unable to pay our liabilities as they become due, or that the realizable value of our assets would, after payment of the dividend, be less than the aggregate value of our liabilities, issued share capital and share premium accounts.

Payment of Additional Amounts

We will make all payments on the Series A Non-Cumulative Preferred Shares free and clear of and without withholding or deduction at source for, or on account of, any present or future taxes, fees, duties, assessments or governmental charges of whatever nature imposed or levied by or on behalf of Bermuda or any other jurisdiction in which we are organized (a "taxing jurisdiction") or any political subdivision or taxing authority thereof or therein, unless such taxes, fees, duties, assessments or governmental charges are required to be withheld or deducted by (x) the laws (or any regulations or rulings promulgated thereunder) of a taxing jurisdiction or any political subdivision or taxing authority thereof or therein or (y) an official position regarding the application, administration, interpretation or enforcement of any such laws, regulations or rulings (including, without limitation, a holding by a court of competent jurisdiction or by a taxing authority in a taxing jurisdiction or any political subdivision thereof). If a withholding or deduction at source is required, we will, subject to certain limitations and exceptions described below, pay to the holders of the Series A Non-Cumulative Preferred Shares such additional amounts as dividends as may be necessary so that every net payment made to such holders, after the withholding or deduction, will not be less than the amount provided for in the Certificate of Designations to be then due and payable.

We will not be required to pay any additional amounts for or on account of:

1. any tax, fee, duty, assessment or governmental charge of whatever nature that would not have been imposed but for the fact that such holder was a resident, citizen, domiciliary or national of, or engaged in business or maintained a permanent establishment or was physically present in, the relevant taxing jurisdiction or any political subdivision thereof or otherwise had some connection with the relevant taxing jurisdiction other than by reason of the mere ownership of, or receipt of payment under, such Series A Non-Cumulative Preferred Shares or any Series A Non-Cumulative Preferred Shares presented for payment more than 30 days after the Relevant Date. The "Relevant Date" means, in respect of any payment, the date on which such payment first becomes due and payable, but if the full amount of the moneys payable has not been received by the dividend disbursing agent on or prior to such due date, it means the first date on which, the

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full amount of such moneys having been so received and being available for payment to holders, notice to that effect shall have been duly given to the holders of the Series A Non-Cumulative Preferred Shares;

2. any estate, inheritance, gift, sale, transfer, personal property or similar tax, assessment or other governmental charge or any tax, assessment or other governmental charge that is payable otherwise than by withholding or deduction from payment of the liquidation preference;

3. any tax, fee, assessment or other governmental charge that is payable otherwise than by withholding or deduction from payment of the liquidation preference of or any dividends on the Series A Non-Cumulative Preferred Shares;

4. any tax, fee, duty, assessment or other governmental charge that is imposed or withheld by reason of the failure by the holder of such Series A Non-Cumulative Preferred Shares to comply with any reasonable request by us addressed to the holder within 90 days of such request (a) to provide information concerning the nationality, citizenship, residence or identity of the holder or (b) to make any declaration or other similar claim or satisfy any information or reporting requirement, which is required or imposed by statute, treaty, regulation or administrative practice of the relevant taxing jurisdiction or any political subdivision thereof as a precondition to exemption from all or part of such tax, fee, duty, assessment or other governmental charge;

5. any withholding or deduction required to be made pursuant to any EU Directive on the taxation of savings implementing the conclusions of the ECOFIN Council meetings of 26-27 November 2000, 3 June 2003 or any law implementing or complying with, or introduced in order to conform to, such EU Directive; or

6. any combination of items (1), (2), (3), (4) and (5).

In addition, we will not pay additional amounts with respect to any payment on any such Series A Non-Cumulative Preferred Shares to any holder who is a fiduciary, partnership, limited liability company or other pass-thru entity other than the sole beneficial owner of such Series A Non-Cumulative Preferred Shares if such payment would be required by the laws of the relevant taxing jurisdiction (or any political subdivision or relevant taxing authority thereof or therein) to be included in the income for tax purposes of a beneficiary or partner or settlor with respect to such fiduciary or a member of such partnership, limited liability company or other pass-thru entity or a beneficial owner to the extent such beneficiary, partner or settlor would not have been entitled to such additional amounts had it been the holder of the Series A Non-Cumulative Preferred Shares.

If we become obligated to pay any additional amounts as a result of a change in tax law, we will also have the option to redeem the Series A Non-Cumulative Preferred Shares. See "—Tax Redemption."

Liquidation Rights

Upon any voluntary or involuntary liquidation, dissolution or winding-up of Arch Capital, holders of the Series A Non-Cumulative Preferred Shares and any parity shares are entitled to receive out of our assets available for distribution to shareholders, after satisfaction of liabilities to creditors, if any, but before any distribution of assets is made to holders of our common shares or any of our other shares ranking junior as to such a distribution to the Series A Non-Cumulative Preferred Shares, a liquidating distribution in the amount of $25 per Series A Non-Cumulative Preferred Share plus any declared and unpaid dividends. If in any such distribution, our assets or proceeds thereof are not sufficient to pay the liquidating distribution, distributions will be made pro rata as to the Series A Non-Cumulative Preferred Shares and any parity shares but only to the extent we have assets available after satisfaction of all liabilities to creditors. Holders of the Series A Non-Cumulative Preferred Shares will not be entitled to any other amounts from us after they have received their full liquidation preference.

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In any such distribution, if our assets are not sufficient to pay the liquidation preferences in full to all holders of the Series A Non-Cumulative Preferred Shares and all holders of any parity shares, the amounts paid to the holders of Series A Non-Cumulative Preferred Shares and to the holders of any parity shares will be paid pro rata in accordance with the respective aggregate liquidation preferences of those holders. In any such distribution, the "liquidation preference" of any holder of preferred shares means the amount payable to such holder in such distribution, including any declared but unpaid dividends (and any unpaid, accrued cumulative dividends in the case of any holder of shares on which dividends accrue on a cumulative basis). If the liquidation preference has been paid in full to all holders of the Series A Non-Cumulative Preferred Shares and any holders of parity shares, the holders of our other shares shall be entitled to receive all of our remaining assets according to their respective rights and preferences.

For purposes of this section, a consolidation, amalgamation, merger, arrangement, reincorporation, de-registration or reconstruction involving Arch Capital or the sale or transfer of all or substantially all of the shares or the property or business of Arch Capital will not be deemed to constitute a liquidation, dissolution or winding-up.

Redemption

The Series A Non-Cumulative Preferred Shares are not subject to any mandatory redemption, sinking fund, retirement fund, purchase fund or other similar provisions. Except as described below under "—Tax Redemption" or as otherwise set forth below, the Series A Non-Cumulative Preferred Shares are not redeemable prior to February 1, 2011. On and after that date, the Series A Non-Cumulative Preferred Shares will be redeemable at our option, in whole or in part, upon not less than 30 days nor more than 60 days notice, at a redemption price equal to $25 per preferred share, plus declared and unpaid dividends, without accumulation of any undeclared dividends. Holders of the Series A Non-Cumulative Preferred Shares will have no right to require the redemption of the Series A Non-Cumulative Preferred Shares.

The Series A Non-Cumulative Preferred Shares are also redeemable at our option at any time prior to February 1, 2011, if we have submitted to the holders of our common shares a proposal for an amalgamation, consolidation, merger, arrangement, reconstruction, reincorporation, de-registration or any other similar transaction involving Arch Capital that requires, or we have submitted any proposal for any other matter that, as a result of any change in Bermuda law after the date of this prospectus supplement (whether by enactment or official interpretation) that requires, in either case, a vote of the holders of the Series A Non-Cumulative Preferred Shares at the time outstanding, voting separately as a single class (alone or with one or more other classes or series of preferred shares). Our option to redeem the Series A Non-Cumulative Preferred Shares under such circumstances shall be for all of the outstanding Series A Non-Cumulative Preferred Shares upon not less than 30 nor more than 60 days prior written notice, and at a redemption price of $26 per Series A Non-Cumulative Preferred Share, plus all declared and unpaid dividends, if any, to the date of redemption, without interest on such unpaid dividends.

The redemption price for any shares of Series A Non-Cumulative Preferred Shares shall be payable on the redemption date to the holders of such shares against book entry transfer or surrender of the certificate(s) evidencing such shares to us or our agent. Any declared but unpaid dividends payable on a redemption date that occurs subsequent to the dividend record date for a dividend period shall not be paid to the holder entitled to receive the redemption price on the redemption date, but rather shall be paid to the holder of record of the redeemed shares on such dividend record date relating to the dividend payment date provided in "—Dividends" above.

Prior to delivering notice of redemption as provided below, we will file with our corporate records a certificate signed by one of our officers affirming our compliance with the redemption provisions

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under the Companies Act 1981 of Bermuda (the "Companies Act") relating to the Series A Non-Cumulative Preferred Shares, and stating that there are reasonable grounds for believing that we are, and after the redemption will be, able to pay our liabilities as they become due and that the redemption will not cause us to breach any provision of applicable Bermuda law or regulation. We will mail a copy of this certificate with the notice of any redemption.

If the Series A Non-Cumulative Preferred Shares are to be redeemed, the notice of redemption shall be given by first class mail to the holders of record of the Series A Non-Cumulative Preferred Shares to be redeemed, mailed not less than 30 days nor more than 60 days prior to the date fixed for redemption thereof (provided that, if the Series A Non-Cumulative Preferred Shares are held in book-entry form through The Depository Trust Company, or "DTC," we may give such notice in any manner permitted by DTC). Each notice of redemption will include a statement setting forth:

•: the redemption date;
•: the number of Series A Non-Cumulative Preferred Shares to be redeemed and, if less than all the Series A Non-Cumulative Preferred Shares held by such holder are to be redeemed, the number of such Series A Non-Cumulative Preferred Shares to be redeemed from such holder;
•: the redemption price; and
•: that the shares should be delivered via book entry transfer or the place or places where holders may surrender certificates evidencing the Series A Non-Cumulative Preferred Shares for payment of the redemption price.

If notice of redemption of any Series A Non-Cumulative Preferred Shares has been given and if the funds necessary for such redemption have been set aside by us for the benefit of the holders of any Series A Non-Cumulative Preferred Shares so called for redemption, then, from and after the redemption date, no further dividends will be declared on such Series A Non-Cumulative Preferred Shares, such Series A Non-Cumulative Preferred Shares shall no longer be deemed outstanding and all rights of the holders of such Series A Non-Cumulative Preferred Shares will terminate, except the right to receive the redemption price, without interest.

In case of any redemption of only part of the Series A Non-Cumulative Preferred Shares at the time outstanding, the Series A Non-Cumulative Preferred Shares to be redeemed shall be selected either pro rata or in such other manner as we may determine to be fair and equitable.

Our ability to redeem the Series A Non-Cumulative Preferred Shares as described above or pursuant to a tax event, as described below under "—Tax Redemption," will be limited by the terms of our agreements governing our indebtedness and by the provisions of other agreements that we may enter into.

Under Bermuda law, we may not redeem our preference shares (including the Series A Non-Cumulative Preferred Shares) at any time if we have reasonable grounds for believing that we are, or after the redemption would be, unable to pay our liabilities as they become due. Preference shares (including the Series A Non-Cumulative Preferred Shares) may not be redeemed except out of the capital paid up thereon or out of our funds that would otherwise be available for dividends or distributions or out of the proceeds of a new issue of shares made for the purpose of the redemption. The premium, if any, payable on redemption must be provided for out of our funds that would otherwise be available for dividends or distributions or out of our share premium account before the preference shares are redeemed. Preference shares also may not be redeemed if as a result of the redemption, our issued share capital would be reduced below the minimum capital specified in the memorandum of association of Arch Capital.

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Tax Redemption

We will have the option to redeem for cash the Series A Non-Cumulative Preferred Shares at any time in whole or from time to time in part, upon not less than 30 days nor more than 60 days prior written notice in accordance with the procedures described under "—Redemption" above, at a redemption price of $25 per share plus declared and unpaid dividends, if any, to the date of redemption, without interest on such unpaid dividends, if as a result of a "change in tax law" there is a substantial probability that we or any successor corporation would be required to pay any additional amounts with respect to the Series A Non-Cumulative Preferred Shares.

A "change in tax law" that would trigger the provisions of the preceding paragraph would be (a) a change in or amendment to laws, regulations or rulings of any jurisdiction, political subdivision or taxing authority described in the next sentence, (b) a change in the official application or interpretation of those laws, regulations or rulings, (c) any execution of or amendment to any treaty affecting taxation to which any jurisdiction, political subdivision or taxing authority described in the next sentence is party after the date of this prospectus supplement, or (d) a decision rendered by a court of competent jurisdiction in Bermuda or any taxing jurisdiction or any political subdivision, whether or not such decision was rendered with respect to us. The jurisdictions, political subdivisions and taxing authorities referred to in the previous sentence are (a) Bermuda or any political subdivision or governmental authority of or in Bermuda with the power to tax, (b) any jurisdiction from or through which we or our dividend disbursing agent are making payments on the Series A Non-Cumulative Preferred Shares or any political subdivision or governmental authority of or in that jurisdiction with the power to tax or (c) any other jurisdiction in which Arch Capital or a successor corporation is organized or generally subject to taxation or any political subdivision or governmental authority of or in that jurisdiction with the power to tax.

In addition, we will have the option to redeem for cash any or all Series A Non-Cumulative Preferred Shares at any time in whole or from time to time in part, upon not less than 30 days nor more than 60 days prior written notice in accordance with the procedures set forth under "—Redemption" above, at a redemption price of $25 per share plus declared and unpaid dividends, if any, to the date of redemption, without interest on such unpaid dividends, if the entity formed by a consolidation, merger or amalgamation involving us or the entity to which we convey, transfer or lease substantially all our properties and assets is required to pay additional amounts in respect of any tax, assessment or governmental charge imposed on any holder of Series A Non-Cumulative Preferred Shares as a result of a change in tax law that occurred after the date of the consolidation, merger, amalgamation, conveyance, transfer or lease.

Voting Rights

Except as provided below, the holders of the Series A Non-Cumulative Preferred Shares will have no voting rights.

Whenever dividends on any Series A Non-Cumulative Preferred Shares shall have not been declared and paid for the equivalent of six or more dividend periods, whether or not for consecutive dividend periods (a "nonpayment event"), the holders of the Series A Non-Cumulative Preferred Shares, voting together as a single class with holders of any and all other series of voting preferred shares (as defined below) then outstanding, will be entitled to vote for the election of a total of two additional members of the board of directors of Arch Capital (the "preferred shares directors"), provided that the election of any such directors shall not cause us to violate the corporate governance requirement of any exchange on which our securities may be listed or quoted that listed or quoted companies must have a majority of independent directors. In that event, the new directors shall be elected at a special meeting called at the request of the holders of record of at least 20% of the aggregate voting power of the Series A Non-Cumulative Preferred Shares or of any other series of

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voting preferred shares (unless such request is received less than 90 days before the date fixed for the next annual or special meeting of the shareholders of Arch Capital, in which event such election shall be held at such next annual or special meeting of shareholders), and at each subsequent annual meeting.

As used in this prospectus supplement, "voting preferred shares" means any other class or series of our preferred shares ranking equally with the Series A Non-Cumulative Preferred Shares as to dividends and the distribution of assets upon liquidation, dissolution or winding-up of Arch Capital and upon which like voting rights have been conferred and are exercisable. Whether a plurality, majority or other portion of the Series A Non-Cumulative Preferred Shares and any other voting preferred shares have been voted in favor of any matter shall be determined by reference to the aggregate voting power, as determined under our bye-laws, of the Series A Non-Cumulative Preferred Shares and voting preferred shares voted.

If and when dividends for at least four dividend periods, whether or not consecutive, following a nonpayment event have been paid in full (or declared and a sum sufficient for such payment shall have been set aside), the holders of the Series A Non-Cumulative Preferred Shares shall be divested of the foregoing voting rights (subject to revesting in the event of each subsequent nonpayment event) and, if such voting rights for all other holders of voting preferred shares have terminated, the term of office of each preferred shares director so elected shall terminate and the number of directors on the board of directors of Arch Capital shall automatically decrease by two. In determining whether dividends have been paid for four dividend periods following a nonpayment event, we may take account of any dividend we elect to pay for such a dividend period after the regular dividend payment date for that period has passed.

Any preferred shares director may be removed at any time without cause by the holders of record of a majority of the aggregate voting power, as determined under our bye-laws, of Series A Non-Cumulative Preferred Shares and any other shares of voting preferred shares then outstanding (voting together as a single class) when they have the voting rights described above. So long as a nonpayment event shall continue, any vacancy in the office of a preferred shares director (other than prior to the initial election after a nonpayment event) may be filled by the written consent of the preferred shares director remaining in office, or if none remains in office, by a vote of the holders of record of a majority of the outstanding Series A Non-Cumulative Preferred Shares and any other shares of voting preferred shares then outstanding (voting together as a single class) when they have the voting rights described above. Any vote of shareholders to remove, or to fill a vacancy in the office of, a preferred shares director may be taken only at a special general meeting of such shareholders, called as provided above for an initial election of preferred shares director after a nonpayment event (unless such request is received less than 90 days before the date fixed for the next annual or special meeting of the shareholders of Arch Capital, in which event such election shall be held at such next annual or special general meeting of shareholders). The preferred shares directors shall each be entitled to one vote per director on any matter. Each preferred shares director elected at any special general meeting of shareholders or by written consent of the other preferred shares director shall hold office until the next annual meeting of the shareholders of Arch Capital if such office shall not have previously terminated as above provided.

The bye-laws of Arch Capital provide that all or any of the special rights attached to any class of shares (including the Series A Non-Cumulative Preferred Shares) issued may, whether or not the Company is being wound-up, be varied with the consent in writing of the holders of a majority of the voting power represented by the issued shares of that class or with the sanction of a resolution passed by a majority of the voting power represented by the votes cast at a separate general meeting of the holders of the shares of the class in accordance with the Companies Act. The bye-laws also provide that rights conferred upon the holders of the shares of any class (including the Series A Non-Cumulative Preferred Shares) issued with preferred or other rights shall not, unless otherwise

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expressly provided by the terms of issue of the shares of that class, be deemed to be varied by the creation or issue of further shares ranking pari passu therewith. The Companies Act provides that in certain circumstances, non-voting shares have the right to vote (for example without limitation, converting a limited liability company to unlimited liability company, discontinuance of a company from Bermuda, or conversion of preference shares into redeemable preference shares).

Notwithstanding the foregoing, our bye-laws contain a provision limiting the voting rights of any U.S. person, as defined in the Internal Revenue Code, who owns (directly, indirectly or constructively under the Code) shares with more than 9.9% of the total voting power of all shares entitled to vote generally at an election of directors to 9.9% of such voting power. See "Certain Income Tax Considerations—Taxation of Shareholders—Controlled Foreign Corporation Rules."

Without the consent of the holders of the Series A Non-Cumulative Preferred Shares, so long as such action does not affect the special rights, preferences, privileges and voting powers of the Series A Non-Cumulative Preferred Shares, taken as a whole, we may amend, alter, supplement or repeal any terms of the Series A Non-Cumulative Preferred Shares:

•: to cure any ambiguity, or to cure, correct or supplement any provision contained in the Certificate of Designations for the Series A Non-Cumulative Preferred Shares that may be defective or inconsistent; or
•: to make any provision with respect to matters or questions arising with respect to the Series A Non-Cumulative Preferred Shares that is not inconsistent with the provisions of the Certificate of Designations.

The foregoing voting provisions will not apply with respect to the Series A Non-Cumulative Preferred Shares if, at or prior to the time when the act with respect to which such vote would otherwise be required shall be effected, all outstanding Series A Non-Cumulative Preferred Shares shall have been redeemed or called for redemption upon proper notice and sufficient funds shall have been set aside by us for the benefit of the holders of Series A Non-Cumulative Preferred Shares to effect such redemption.

Conversion

The Series A Non-Cumulative Preferred Shares are not convertible into or exchangeable for any other securities or property of Arch Capital.

Capital Replacement

We intend that, if we redeem the Series A Non-Cumulative Preferred Shares, we will redeem such Series A Non-Cumulative Preferred Shares only to the extent the aggregate redemption price is equal to or less than the net proceeds, if any, received by us or any of our affiliates from new issuances by us or any of our affiliates during the period commencing on the 180th calendar day prior to the date of redemption to purchasers other than our affiliates (other than persons who are affiliates of Arch Capital as a result of their ownership of common stock of Arch Capital) of any securities which have equal or greater equity characteristics for us as the Series A Non-Cumulative Preferred Shares.

Listing of the Series A Non-Cumulative Preferred Shares

We intend to apply to have the Series A Non-Cumulative Preferred Shares approved for listing on the NYSE.

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Transfer Agent, Registrar, Dividend Disbursing Agent and Redemption Agent

American Stock Transfer & Trust Company will be the transfer agent, registrar, dividend disbursing agent and redemption agent for the Series A Non-Cumulative Preferred Shares.

Book-Entry; Delivery and Form

The Series A Non-Cumulative Preferred Shares will be represented by one or more global securities that will be deposited with and registered in the name of DTC or its nominee. This means that we will not issue certificates to you for the Series A Non-Cumulative Preferred Shares except in limited circumstances. The global securities will be issued to DTC, the depository for the Series A Non-Cumulative Preferred Shares, who will keep a computerized record of its participants (for example, your broker) whose clients have purchased the Series A Non-Cumulative Preferred Shares. Each participant will then keep a record of its clients. Unless exchanged in whole or in part for a certificated security, a global security may not be transferred. However, DTC, its nominees, and their successors may transfer a global security as a whole to one another. Beneficial interests in the global securities will be shown on, and transfers of the global securities will be made only through, records maintained by DTC and its participants.

DTC has advised us that it is a limited-purpose trust company organized under the New York Banking Law, a "banking organization" within the meaning of the New York Banking Law, a member of the United States Federal Reserve System, a "clearing corporation" within the meaning of the New York Uniform commercial Code and a "clearing agency" registered under the provisions of Section 17A of the Securities Exchange Act of 1934, as amended. DTC holds securities that its participants (direct participants) deposit with DTC. DTC also records the settlement among direct participants of securities transactions, such as transfers and pledges, in deposited securities through computerized records for direct participants' accounts. This eliminates the need to exchange certificates. Direct participants include securities brokers and dealers, banks, trust companies, clearing corporations and certain other organizations. Neither we nor the underwriters take any responsibility for these operations or procedures, and you are urged to contact DTC or its participants directly to discuss these matters.

DTC's book-entry system is also used by other organizations such as securities brokers and dealers, banks and trust companies that work through a direct participant. The rules that apply to DTC and its participants are on file with the SEC.

DTC is owned by a number of its direct participants and by the New York Stock Exchange, Inc., the American Stock Exchange, Inc. and the National Association of Securities Dealers, Inc.

When you purchase Series A Non-Cumulative Preferred Shares through the DTC system, the purchases must be made by or through a direct participant, who will receive credit for the Series A Non-Cumulative Preferred Shares on DTC's records. Since you actually own the Series A Non-Cumulative Preferred Shares, you are the beneficial owner and your ownership interest will only be recorded in the direct (or indirect) participants' records. DTC has no knowledge of your individual ownership of the Series A Non-Cumulative Preferred Shares. DTC's records only show the identity of the direct participants and the amount of the Series A Non-Cumulative Preferred Shares held by or through them. You will not receive a written confirmation of your purchase or sale or any periodic account statement directly from DTC. You will receive these from your direct (or indirect) participant. Thus, the direct (or indirect) participants are responsible for keeping accurate account of the holdings of their customers like you.

We will wire dividend payments to DTC's nominee and we will treat DTC's nominee as the owner of the global securities for all purposes. Accordingly, we will have no direct responsibility or liability to pay amounts due on the global securities to you or any other beneficial owners in the global securities.

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Any redemption notices will be sent by us directly to DTC, who will in turn inform the direct participants, who will then contact you as a beneficial holder.

It is DTC's current practice, upon receipt of any payment of dividends or liquidation amount, to credit direct participants' accounts on the payment date based on their holdings of beneficial interests in the global securities as shown on DTC's records. In addition, it is DTC's current practice to assign any consenting or voting rights to direct participants whose accounts are credited with preferred securities on a record date, by using an omnibus proxy. Payments by participants to owners of beneficial interests in the global securities, and voting by participants, will be based on the customary practices between the participants and owners of beneficial interests, as is the case with the Series A Non-Cumulative Preferred Shares held for the account of customers registered in "street name." However, payments will be the responsibility of the participants and not of DTC or us.

Series A Non-Cumulative Preferred Shares represented by global securities will be exchangeable for certificated securities with the same terms in authorized denominations only if:

•: DTC is unwilling or unable to continue as depositary or if DTC ceases to be a clearing agency registered under applicable law and a successor depositary is not appointed by us within 90 days; or
•: we determine not to require all of the Series A Non-Cumulative Preferred Shares to be represented by global securities.

If the book-entry-only system is discontinued, the transfer agent will keep the registration books for the Series A Non-Cumulative Preferred Shares at its corporate office.

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CERTAIN INCOME TAX CONSIDERATIONS

The following summary of the taxation of ACGL and the taxation of our shareholders is based upon current law and is for general information only. Legislative, judicial or administrative changes may be forthcoming that could affect this summary.

The following legal discussion (including and subject to the matters and qualifications set forth in such summary) of certain tax considerations (a) under "—Taxation of ACGL—Bermuda" and "—Taxation of Shareholders—Bermuda Taxation" is based upon the advice of Conyers Dill & Pearman, Hamilton, Bermuda and (b) under "—Taxation of ACGL—United States," "—Taxation of Shareholders—United States Taxation," and "—United States Taxation of Non-U.S. Shareholders" is based upon the advice of Cahill Gordon & Reindel LLP, New York, New York (the advice of such firms does not include accounting matters, determinations or conclusions relating to the business or activities of ACGL). The summary is based upon current law and is for general information only. The tax treatment of a holder of our Series A Non-Cumulative Preferred Shares, or of a person treated as a holder of our Series A Non-Cumulative Preferred Shares for U.S. federal income, state, local or non-U.S. tax purposes, may vary depending on the holder's particular tax situation. Legislative, judicial or administrative changes or interpretations may be forthcoming that could be retroactive and could affect the tax consequences to us or to holders of our Series A Non-Cumulative Preferred Shares.

Taxation of ACGL

Bermuda

Under current Bermuda law, we are not subject to tax on income or capital gains. We have obtained from the Minister of Finance under the Exempted Undertakings Tax Protection Act 1966 an assurance that, in the event that Bermuda enacts legislation imposing tax computed on profits, income, any capital asset, gain or appreciation, or any tax in the nature of estate duty or inheritance, the imposition of any such tax shall not be applicable to us or to any of our operations or our shares, debentures or other obligations until March 28, 2016. We could be subject to taxes in Bermuda after that date. This assurance will be subject to the proviso that it is not to be construed so as to prevent the application of any tax or duty to such persons as are ordinarily resident in Bermuda (we are not so currently affected) or to prevent the application of any tax payable in accordance with the provisions of the Land Tax Act 1967 or otherwise payable in relation to any property leased to us or our insurance subsidiary. We pay annual Bermuda government fees, and our Bermuda insurance and reinsurance subsidiary pays annual insurance license fees. In addition, all entities employing individuals in Bermuda are required to pay a payroll tax and other sundry other taxes payable, directly or indirectly, to the Bermuda government.

United States

ACGL and its non-U.S. subsidiaries intend to conduct their operations in a manner that will not cause them to be treated as engaged in a trade or business in the United States and, therefore, will not be required to pay U.S. federal income taxes (other than U.S. excise taxes on insurance and reinsurance premium and withholding taxes on dividends and certain other U.S. source investment income). However, because definitive identification of activities which constitute being engaged in a trade or business in the United States is not provided by the Code, or regulations or court decisions, there can be no assurance that the U.S. Internal Revenue Service will not contend successfully that ACGL or its non-U.S. subsidiaries are or have been engaged in a trade or business in the United States. A foreign corporation deemed to be so engaged would be subject to U.S. income tax, as well as the branch profits tax, on its income, which is treated as effectively connected with the conduct of that trade or business unless the corporation is entitled to relief under the permanent establishment provisions of a tax treaty. Such income tax, if imposed, would be based on effectively connected income

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computed in a manner generally analogous to that applied to the income of a domestic corporation, except that deductions and credits generally are not permitted unless the foreign corporation has timely filed a U.S. federal income tax return in accordance with applicable regulations. Penalties may be assessed for failure to file tax returns. The 30% branch profits tax is imposed on net income after subtracting the regular corporate tax and making certain other adjustments.

Under the income tax treaty between Bermuda and the United States (the "Treaty"), ACGL's Bermuda insurance subsidiaries will be subject to U.S. income tax on any insurance premium income found to be effectively connected with a U.S. trade or business only if that trade or business is conducted through a permanent establishment in the United States. No regulations interpreting the Treaty have been issued. While there can be no assurances, ACGL does not believe that any of its Bermuda insurance subsidiaries has a permanent establishment in the United States. Such subsidiaries would not be entitled to the benefits of the Treaty if (i) less than 50% of ACGL's shares were beneficially owned, directly or indirectly, by Bermuda residents or U.S. citizens or residents, or (ii) any such subsidiary's income were used in substantial part to make disproportionate distributions to, or to meet certain liabilities to, persons who are not Bermuda residents or U.S. citizens or residents. While there can be no assurances, ACGL believes that its Bermuda insurance subsidiaries are eligible for Treaty benefits.

Foreign insurance companies carrying on an insurance business within the United States have a certain minimum amount of effectively connected net investment income, determined in accordance with a formula that depends, in part, on the amount of U.S. risk insured or reinsured by such companies. If any of ACGL's non-U.S. insurance subsidiaries is considered to be engaged in the conduct of an insurance business in the United States, a significant portion of such company's investment income could be subject to U.S. income tax.

Foreign corporations not engaged in a trade or business in the United States are nonetheless subject to U.S. income tax on certain "fixed or determinable annual or periodic gains, profits and income" derived from sources within the United States as enumerated in Section 881(a) of the Code (such as dividends and certain interest on investments), subject to exemption under the Code or reduction by applicable treaties.

The United States also imposes an excise tax on insurance and reinsurance premiums paid to foreign insurers or reinsurers with respect to risks located in the United States. The rates of tax applicable to premiums are 4% for casualty insurance premiums and 1% for reinsurance premiums.

Personal Holding Company Rules. A domestic corporation will not be classified as a personal holding company (a "PHC") in a given taxable year unless both (i) at some time during the last half of such taxable year, five or fewer individuals (without regard to their citizenship or residency) own or are deemed to own (pursuant to certain constructive ownership rules) more than 50% of the corporation's shares by value, and (ii) at least 60% of the adjusted ordinary gross income of the corporation for such taxable year consists of PHC income (as defined in Section 543 of the Code). For purposes of the 50% share ownership test, all of our shares owned by an investment partnership will be attributed to each of its partners, if any, who are individuals. As a result of this attribution rule, we believe that currently five or fewer individuals are treated as owning more than 50% of the value of our shares. Consequently, one or more of our domestic subsidiaries could be or become PHCs, depending on whether any of our subsidiaries satisfy the PHC gross income test.

We will use reasonable best efforts to cause each of our domestic subsidiaries not to satisfy the gross income requirement set forth in Section 542(a) of the Code. If, however, any of our domestic subsidiaries is or were to become a PHC in a given taxable year, such company would be subject to PHC tax (at a 15% rate for taxable years before January 1, 2009, and thereafter at the highest marginal rate on ordinary income applicable to individuals) on its "undistributed PHC income." PHC income

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generally would not include underwriting income. If any of our subsidiaries is or becomes a PHC, there can be no assurance that the amount of PHC income would be immaterial.

Certain of our U.S. subsidiaries have been PHCs. Such subsidiaries did not have "undistributed personal holding company income" and do not expect to have "undistributed personal holding company income" in 2006.

There can be no assurance that each of our domestic subsidiaries are not or will not become a PHC in the future because of factors including factual uncertainties regarding the application of the PHC rules, the makeup of our shareholder base and other circumstances that affect the application of the PHC rules to our domestic subsidiaries.

United Kingdom

Our European subsidiaries, Arch-Europe and Arch Capital UK Ltd., are companies incorporated in the U.K. and are therefore resident in the U.K. for U.K. corporation tax purposes and will be subject to U.K. corporate tax in their respective worldwide profits. The current rate of U.K. corporation tax is generally 30% on profits. Currently, U.K. withholding tax applies to dividends that Arch-Europe may pay.

Taxation of Shareholders

The following summary sets forth certain United States federal income tax considerations related to the purchase, ownership and disposition of our Series A Non-Cumulative Preferred Shares. Unless otherwise stated, this summary deals only with shareholders ("U.S. Holders") that are United States Persons (as defined below) who hold their Series A Non-Cumulative Preferred Shares as capital assets and as beneficial owners. The following discussion is only a general summary of the United States federal income tax matters described herein and does not purport to address all of the United States federal income tax consequences that may be relevant to a particular shareholder in light of such shareholder's specific circumstances. In addition, the following summary does not describe the United States federal income tax consequences that may be relevant to certain types of shareholders, such as banks, insurance companies, regulated investment companies, real estate investment trusts, financial asset securitization investment trusts, dealers in securities or traders that adopt a mark-to-market method of tax accounting, tax exempt organizations, persons owning (directly, indirectly or constructively) 10% or more of the total combined voting power or total value of the stock of ACGL, expatriates or persons who hold the Series A Non-Cumulative Preferred Shares as part of a hedging or conversion transaction or as part of a straddle, who may be subject to special rules or treatment under the Code. This discussion is based upon the Code, the Treasury regulations promulgated thereunder and any relevant administrative rulings or pronouncements or judicial decisions, all as in effect on the date of this prospectus supplement and as currently interpreted, and does not take into account possible changes in such tax laws or interpretations thereof, which may apply retroactively. This discussion does not include any description of the alternative minimum tax or the tax laws of any state or local governments within the United States, or of any foreign government, that may be applicable to the Series A Non-Cumulative Preferred Shares or the shareholders. Persons considering making an investment in the Series A Non-Cumulative Preferred Shares should consult their own tax advisors concerning the application of the United States federal tax laws to their particular situations as well as any tax consequences arising under the laws of any state, local or foreign taxing jurisdiction prior to making such investment.

If a partnership holds our Series A Non-Cumulative Preferred Shares, the tax treatment of a partner will generally depend upon the status of the partner and the activities of the partnership. If you are a partner of a partnership holding Series A Non-Cumulative Preferred Shares, you should consult your tax advisor.

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For purposes of this discussion, the term "United States Person" means:

•: a citizen or resident of the United States,
•: a corporation or entity treated as a corporation created or organized in or under the laws of the United States, or any political subdivision thereof,
•: an estate the income of which is subject to United States federal income taxation regardless of its source,
•: a trust if either (x) a court within the United States is able to exercise primary supervision over the administration of such trust and one or more United States Persons have the authority to control all substantial decisions of such trust or (y) the trust has a valid election in effect to be treated as a United States Person for U.S. federal income tax purposes or
•: any other person or entity that is treated for U.S. federal income tax purposes as if it were one of the foregoing.

Bermuda Taxation

Currently, there is no Bermuda withholding tax on dividends paid by us.

United States Taxation

Taxation of Dividends. The Series A Non-Cumulative Preferred Shares shall be properly classified as equity rather than debt for U.S. federal income tax purposes. Subject to the discussions below relating to the potential application of the CFC, RPII and PFIC rules, cash distributions, if any, made with respect to our Series A Non-Cumulative Preferred Shares will constitute dividends for U.S. federal income tax purposes to the extent paid out of our current or accumulated earnings and profits (as computed using U.S. tax principles). If a U.S. Holder of our Series A Non-Cumulative Preferred Shares is an individual or other non-corporate holder, dividends paid, if any, to that holder in taxable years beginning before January 1, 2009 that constitute qualified dividend income will be taxable at the rate applicable for long-term capital gains (generally up to 15%), provided that such person meets a holding period requirement. Generally in order to meet the holding period requirement, the United States Person must hold the Series A Non-Cumulative Preferred Shares for more than 90 days during the 181-day period beginning 90 days before the ex-dividend date. Dividends paid, if any, with respect to Series A Non-Cumulative Preferred Shares generally will be qualified dividend income, provided the Series A Non-Cumulative Preferred Shares are classified as equity rather than debt for U.S. federal income tax purposes (which should be the case), the Series A Non-Cumulative are considered readily tradable on an established securities market in the United States in the year in which the shareholder receives the dividend (which should be the case if, as is intended, such shares are listed on the NYSE) and ACGL is not considered to be a passive foreign investment company or "PFIC" in either the year of the distribution or the preceding taxable year. No assurance can be given that the Series A Non-Cumulative Preferred will be so listed or otherwise be considered readily tradable on an established securities market in the United States. There is a risk that dividends, if any, paid prior to the listing of the Series A Non-Cumulative Preferred Shares on the NYSE may not constitute qualified dividend income.

After December 31, 2008, qualified dividend income will no longer be taxed at the rate applicable for long-term capital gains unless Congress enacts legislation providing otherwise. A legislative proposal passed by the Ways and Means Committee in the House of Representatives would extend the preferential treatment of qualified dividend income to dividends paid prior to December 31, 2010, but this proposal has not been included in the Senate-passed Tax Reconciliation Bill, and no assurances can be given that the proposal will be enacted. A legislative proposal introduced in the Senate would increase the holding period requirement for dividends paid on preferred stock. While this proposal has

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not been included in the Senate-passed Tax Reconciliation Bill, no assurances can be given that this or a similar proposal will not be enacted.

Distributions with respect to the Series A Non-Cumulative Preferred Shares will not be eligible for the dividends-received deduction allowed to U.S. corporations under the Code. To the extent distributions on our Series A Non-Cumulative Preferred Shares exceed our earnings and profits, they will be treated first as a return of the U.S. Holder's basis in our Series A Non-Cumulative Preferred Shares to the extent thereof, and then as gain from the sale of a capital asset.

Sale, Exchange or Other Disposition. Subject to the discussions below relating to the potential application of the CFC, RPII and PFIC rules, holders of Series A Non-Cumulative Preferred Shares generally will recognize capital gain or loss for U.S. federal income tax purposes on the sale, exchange or disposition of Series A Non-Cumulative Preferred Shares.

Redemption of Series A Non-Cumulative Preference Shares. A redemption of the Series A Non-Cumulative Preferred Shares will be treated under section 302 of the Code as a dividend if we have sufficient earnings and profits, unless the redemption satisfies one of the tests set forth in section 302(b) of the Code enabling the redemption to be treated as a sale or exchange, subject to the discussion herein relating to the potential application of the CFC, RPII and PFIC rules. Under the relevant Code section 302(b) tests, the redemption should be treated as a sale or exchange only if it (1) is substantially disproportionate, (2) constitutes a complete termination of the holder's stock interest in us or (3) is "not essentially equivalent to a dividend." In determining whether any of these tests are met, shares considered to be owned by the holder by reason of certain constructive ownership rules set forth in the Code, as well as shares actually owned, must generally be taken into account. It may be more difficult for a United States Person who owns, actually or constructively by operation of the attribution rules, any of our other shares to satisfy any of the above requirements. The determination as to whether any of the alternative tests of section 302(b) of the Code is satisfied with respect to a particular holder of the preference shares depends on the facts and circumstances as of the time the determination is made.

Controlled Foreign Corporation Rules

Under our bye-laws, the 9.9% voting restriction applicable to the Controlled Shares of a U.S. Person (as defined in our bye-laws) generally does not apply to certain of our investors. As a result of certain attribution rules, we believe, therefore, that we and our foreign subsidiaries are controlled foreign corporations ("CFCs"). That status as a CFC does not cause us or any of our subsidiaries to be subject to U.S. federal income tax. Such status also has no adverse U.S. federal income tax consequences for any U.S. Holder that is considered to own less than 10% of the total combined voting power of our shares or those of our foreign subsidiaries. Only U.S. Holders that are considered to own 10% or more of the total combined voting power of our shares or those of our foreign subsidiaries (taking into account shares actually owned by such U.S. Holder as well as shares attributed to such U.S. Holder under the Code or the regulations thereunder) (a "10% U.S. Voting Shareholder") on the last day of the CFC's taxable year is affected by our status as a CFC. The Series A Non-Cumulative Preferred Shares generally should not be considered voting stock for purposes of determining whether a United States Person would be a "10% U.S. Voting Shareholder." The Series A Non-Cumulative Preferred Shares may, however, become entitled to vote (as a class along with any other class of preferred shares of ACGL then outstanding) for the election of two additional members of the board of directors of ACGL if ACGL does not declare and pay dividends for the equivalent of six or more dividend periods. In such case, the Series A Non-Cumulative Preferred Shares should be treated as voting stock for as long as such voting rights continue. Our bye-laws are intended to prevent any U.S. Holder from being considered a 10% U.S. Voting Shareholder by limiting the votes conferred by the Controlled Shares (as defined in our bye-laws) of any U.S. Person to 9.9% of the total voting power of all our shares entitled to vote. However, because under our bye-laws certain funds associated with

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Warburg Pincus and Hellman & Friedman generally are entitled to vote their directly owned shares in full, a U.S. Holder that is attributed (under the Code or the regulations thereunder) shares owned by such funds may be considered a 10% U.S. Voting Shareholder. If you are a direct or indirect investor in a fund associated with Warburg Pincus or Hellman & Friedman additional shares could be attributed to you for purposes of determining whether you are considered to be a 10% U.S. Voting Shareholder. As long as we are a CFC, a U.S. Holder that is considered a 10% U.S. Voting Shareholder will be subject to current U.S. federal income taxation (at ordinary income tax rates) to the extent of all or a portion of the undistributed earnings and profits of ACGL and our subsidiaries attributable to "subpart F income" (including certain insurance premium income and investment income) and may be taxable at ordinary income tax rates on any gain realized on a sale or other disposition (including by way of repurchase or liquidation) of our shares to the extent of the current and accumulated earnings and profits attributable to such shares.

While our bye-laws include provisions that are intended to prevent any member from being considered a 10% U.S. Voting Shareholder (except as described above), there can be no assurance that a U.S. Holder will not be treated as a 10% U.S. Voting Shareholder, by attribution or otherwise, under the Code or any applicable regulations thereunder. See "Risk Factors—Risks Relating to Taxation—U.S. persons who hold our Series A Non-Cumulative Preferred Shares may be subject to U.S. income taxation at ordinary income rates on our undistributed earnings and profits."

Related Person Insurance Income Rules

Generally, we do not expect the gross "related person insurance income" ("RPII") of any of our wholly-owned non-U.S. insurance subsidiaries to equal or exceed 20% of its gross insurance income in any taxable year for the foreseeable future and do not expect the direct or indirect insureds (and related persons) of any such subsidiary to directly or indirectly own 20% or more of either the voting power or value of our stock. We do not expect any U.S. person owning Series A Non-Cumulative Preferred Shares to be required to include in gross income for U.S. federal income tax purposes RPII income, but there can be no assurance that this will be the case.

Section 953(c)(7) of the Code generally provides that Section 1248 of the Code (which generally would require a U.S. Holder that is a 10% U.S. Voting Shareholder to treat certain gains attributable to the sale, exchange or disposition of Series A Non-Cumulative Preferred Shares as a dividend) will apply to the sale or exchange by a U.S. shareholder of shares in a foreign corporation that is characterized as a CFC under the RPII rules if the foreign corporation would be taxed as an insurance company if it were a domestic corporation, regardless of whether the U.S. shareholder is a 10% U.S. Voting Shareholder or whether the corporation qualifies for either the RPII 20% ownership exception or the RPII 20% gross income exception. Although existing Treasury Department regulations do not address the issue, proposed Treasury regulations issued in April 1991 create some ambiguity as to whether Section 1248 and the requirement to file Form 5471 would apply when the foreign corporation has a foreign insurance subsidiary that is a CFC for RPII purposes and that would be taxed as an insurance company if it were a domestic corporation. We believe that Section 1248 and the requirement to file Form 5471 will not apply to a less than 10% U.S. Voting Shareholder because ACGL is not directly engaged in the insurance business. There can be no assurance, however, that the U.S. Internal Revenue Service will interpret the proposed regulations in this manner or that the Treasury Department will not take the position that Section 1248 and the requirement to file Form 5471 will apply to dispositions of our Series A Non-Cumulative Preferred Shares.

If the U.S. Internal Revenue Service or U.S. Treasury Department were to make Section 1248 and the Form 5471 filing requirement applicable to the sale of our Series A Non-Cumulative Preferred Shares, we would notify shareholders that Section 1248 of the Code and the requirement to file Form 5471 will apply to dispositions of our Series A Non-Cumulative Preferred Shares. Thereafter, we would send a notice after the end of each calendar year to all persons who were shareholders during

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the year notifying them that Section 1248 and the requirement to file Form 5471 apply to dispositions of our Series A Non-Cumulative Preferred Shares by U.S. Holders. We would attach to this notice a copy of Form 5471 completed with all our information and instructions for completing the shareholder information.

Tax-Exempt Shareholders

Tax-exempt entities may be required to treat certain Subpart F insurance income, including RPII, that is includible in income by the tax-exempt entity as unrelated business taxable income. Prospective investors that are tax exempt entities are urged to consult their tax advisors as to the potential impact of the unrelated business taxable income provisions of the Code.

Passive Foreign Investment Companies

Sections 1291 through 1298 of the Code contain special rules applicable with respect to foreign corporations that are "passive foreign investment companies" ("PFICs"). In general, a foreign corporation will be a PFIC if 75% or more of its income constitutes "passive income" or 50% or more of its assets produce passive income. If we were to be characterized as a PFIC, U.S. Holders would be taxable at ordinary income tax rates and subject to a penalty tax at the time of their sale or disposition at a gain of (or receipt of an "excess distribution" with respect to) their Series A Non-Cumulative Preferred Shares. In addition, a dividend paid by a PFIC is not eligible for the reduced rate of tax on qualified dividend income. In general, a shareholder receives an "excess distribution" if the amount of the distribution is more than 125% of the average distribution with respect to the shares during the three preceding taxable years (or shorter period during which the taxpayer held the stock). In general, the penalty tax is equivalent to an interest charge on taxes that are deemed due during the period the shareholder owned the shares, computed by assuming that the excess distribution or gain (in the case of a sale or other disposition) with respect to the shares was taxable in equal portions throughout the holder's period of ownership. The interest charge is equal to the applicable rate imposed on underpayments of U.S. federal income tax for such period. A U.S. shareholder may avoid some of the adverse tax consequences of owning shares in a PFIC by making a qualified electing fund ("QEF") election or "mark-to-market" election. A QEF election is revocable only with the consent of the IRS and has the following consequences to a shareholder:

•: For any year in which ACGL is not a PFIC, no income tax consequences would result.
•: For any year in which the ACGL is a PFIC, the shareholder would include in its taxable income a proportionate share of the net ordinary income and net capital gains of ACGL and certain of its non-U.S. subsidiaries.

The PFIC statutory provisions contain an express exception for income "derived in the active conduct of an insurance business by a corporation which is predominantly engaged in an insurance business.. ." This exception is intended to ensure that income derived by a bona fide insurance company is not treated as passive income, except to the extent such income is attributable to financial reserves in excess of the reasonable needs of the insurance business. The PFIC statutory provisions contain a look-through rule that states that, for purposes of determining whether a foreign corporation is a PFIC, such foreign corporation shall be treated as if it "received directly its proportionate share of the income" and as if it "held its proportionate share of the assets" of any other corporation in which it owns at least 25% of the stock. We believe that we are not a PFIC, and we will use reasonable best efforts to cause us and each of our non-U.S. insurance subsidiaries not to constitute a PFIC.

No regulations interpreting the substantive PFIC provisions have yet been issued. Each U.S. Holder should consult his tax advisor as to the effects of these rules.

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Information Reporting and Backup Withholding

Unless a U.S. Holder is an exempt recipient, such as a corporation, payments on Series A Non-Cumulative Preferred Shares and the proceeds received from the sale of Series A Non-Cumulative Preferred Shares may be subject to information reporting and may also be subject to U.S. federal backup withholding tax if such U.S. Holder fails to supply an accurate taxpayer identification number or other wise fails to comply with applicable U.S. information reporting or certification requirements. Any amounts so withheld generally will be allowed as a credit against the U.S. Holder's U.S. federal income tax liability (and may entitle such holder to a refund), provided that the required information is timely furnished to the IRS.

United States Taxation of Non-U.S. Shareholders

Taxation of Dividends

Cash distributions, if any, made with respect to Series A Non-Cumulative Preferred Shares held by shareholders who are not United States Persons ("Non-U.S. holders") generally will not be subject to United States withholding tax.

Sale, Exchange or Other Disposition

Non-U.S. holders of Series A Non-Cumulative Preferred Shares generally will not be subject to U.S. federal income tax with respect to gain realized upon the sale, exchange or other disposition of Series A Non-Cumulative Preferred Shares unless such gain is effectively connected with a U.S. trade or business of the Non-U.S. holder in the United States or such person is present in the United States for 183 days or more in the taxable year the gain is realized and certain other requirements are satisfied.

Information Reporting and Backup Withholding

Non-U.S. holders of Series A Non-Cumulative Preferred Shares will not be subject to U.S. information reporting or backup withholding with respect to dispositions of Series A Non-Cumulative Preferred Shares effected through a non-U.S. office of a broker, unless the broker has certain connections to the United States or is a United States person. No U.S. backup withholding will apply to payments of dividends, if any, on our Series A Non-Cumulative Preferred Shares.

Other Tax Laws

Shareholders should consult their own tax advisors with respect to the applicability to them of the tax laws of other jurisdictions.

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UNDERWRITING

We intend to offer the Series A Non-Cumulative Preferred Shares in the U.S. through the underwriters. Merrill Lynch, Pierce, Fenner & Smith Incorporated ("Merrill Lynch") and J.P. Morgan Securities Inc. ("JPMorgan") are acting as representatives of the underwriters named below. Subject to the terms and conditions described in a purchase agreement among us and the underwriters, we have agreed to sell to the underwriters, and the underwriters severally have agreed to purchase from us, the number of shares listed opposite their names below.

Underwriter		Number of Series A Non-Cumulative Preferred Shares
Merrill Lynch, Pierce, Fenner & Smith Incorporated		1,187,000
J.P. Morgan Securities Inc.		1,187,000
Citigroup Global Markets Inc.		1,186,500
Goldman, Sachs & Co.		1,186,500
UBS Securities LLC		1,186,500
Wachovia Capital Markets, LLC		1,186,500
Credit Suisse Securities (USA) LLC		160,000
Advest, Inc.		90,000
KeyBanc Capital Markets		90,000
Oppenheimer & Co. Inc.		90,000
Piper Jaffray & Co.		90,000
RBC Dain Rauscher Inc.		90,000
BB&T Capital Markets		30,000
D.A. Davidson & Co.		30,000
Ferris, Baker Watts, Incorporated		30,000
Janney Montgomery Scott LLC		30,000
Keefe, Bruyette & Woods, Inc.		30,000
Morgan Keegan & Company, Inc.		30,000
Ryan Beck & Co., Inc.		30,000
Stifel, Nicolaus & Company, Incorporated		30,000
B.C. Ziegler and Company		30,000

	Total	8,000,000

The underwriters have agreed to purchase all of the Series A Non-Cumulative Preferred Shares sold under the purchase agreement if any of these Series A Non-Cumulative Preferred Shares are purchased. If an underwriter defaults, the purchase agreement provides that the purchase commitments of the nondefaulting underwriters may be increased or the purchase agreement may be terminated. The closings for the sale of Series A Non-Cumulative Preferred Shares to be purchased by the underwriters are conditioned on one another.

We have agreed to indemnify the underwriters against certain liabilities, including liabilities under the Securities Act, or to contribute to payments the underwriters may be required to make in respect of those liabilities.

The underwriters are offering the Series A Non-Cumulative Preferred Shares, subject to prior sale, when, as and if issued to and accepted by them, subject to approval of legal matters by their counsel, including the validity of the shares, and other conditions contained in the purchase agreement, such as the receipt by the underwriters of officer's certificates and legal opinions. The underwriters reserve the right to withdraw, cancel or modify offers to the public and to reject orders in whole or in part.

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Commissions and Discounts

The representatives have advised us that the underwriters propose initially to offer the Series A Non-Cumulative Preferred Shares to the public at the public offering price on the cover page of this prospectus supplement and to dealers at that price less a concession not in excess of $0.50 per Series A Non-Cumulative Preferred Share. The underwriters may allow, and the dealers may reallow, a discount not in excess of $0.45 per Series A Non-Cumulative Preferred Share to other dealers. After commencement of this offering, the public offering price, concessions and discount may be changed.

The following table shows the public offering price, underwriting discount and proceeds before expenses to ACGL.

	Per Share		Total
Public offering price	$	25.0000	$	200,000,000
Underwriting discount	$	0.7875	$	6,300,000
Proceeds, before expenses, to ACGL	$	24.2125	$	193,700,000

The expenses of the offering, not including the underwriting discount, are estimated at approximately $1 million and are payable by ACGL.

No Sales of Similar Securities

We have agreed, with exceptions, not to sell or issue any Series A Non-Cumulative Preferred Shares (or any securities convertible into Series A Non-Cumulative Preferred Shares) for 45 days after the date of this prospectus supplement without first obtaining the written consent of Merrill Lynch and JPMorgan. Specifically, we have agreed with Merrill Lynch and JPMorgan not to directly or indirectly:

•: offer, pledge, sell, contract to sell, sell any option or contract to purchase, grant any option, right or warrant for the sale of, or otherwise dispose of or transfer any Series A Non-Cumulative Preferred Shares (or any securities convertible into Series A Non-Cumulative Preferred Shares), or
•: enter into any swap or any other agreement or any transaction that transfers, in whole or in part, directly or indirectly, the economic consequence of ownership of any Series A Non-Cumulative Preferred Shares, whether any such swap or transaction is to be settled by delivery of Series A Non-Cumulative Preferred Shares or other securities, in cash or otherwise.

This lockup provision applies to Series A Non-Cumulative Preferred Shares and to securities convertible into or exchangeable or exercisable for Series A Non-Cumulative Preferred Shares.

Listing on the NYSE

We intend to apply to have the Series A Non-Cumulative Preferred Shares approved for listing on the NYSE. The Series A Non-Cumulative Preferred Shares are a new issue of securities with no established trading market. The underwriters have advised us that they presently intend to make a market in the Series A Non-Cumulative Preferred Shares. However, they are under no obligation to do so and may discontinue any market-making activities at any time without any notice. We cannot assure the liquidity of the trading market for the Series A Non-Cumulative Preferred Shares or that an active public market for the Series A Non-Cumulative Preferred Shares will develop. If an active public trading market for the Series A Non-Cumulative Preferred Shares does not develop, the market price and liquidity of the Series A Non-Cumulative Preferred Shares may be adversely affected. If the Series A Non-Cumulative Preferred Shares are traded, they may trade at a discount from the initial offering price, depending on prevailing interest rates, the market for similar securities, our performance and other factors.

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Price Stabilization, Short Positions

Until the distribution of the Series A Non-Cumulative Preferred Shares is completed, SEC rules may limit underwriters and selling group members from bidding for and purchasing our Series A Non-Cumulative Preferred Shares. However, the representatives may engage in transactions that stabilize the price of the Series A Non-Cumulative Preferred Shares, such as bids or purchases to peg, fix or maintain that price.

If the underwriters create a short position in the Series A Non-Cumulative Preferred Shares in connection with the offering, i.e., if they sell more Series A Non-Cumulative Preferred Shares than are listed on the cover of this prospectus supplement, the representatives may reduce that short position by purchasing Series A Non-Cumulative Preferred Shares in the open market. Purchases of the Series A Non-Cumulative Preferred Shares to stabilize its price or to reduce a short position may cause the price of the Series A Non-Cumulative Preferred Shares to be higher than it might be in the absence of such purchases.

Neither we nor any of the underwriters makes any representation or prediction as to the direction or magnitude of any effect that the transactions described above may have on the price of the Series A Non-Cumulative Preferred Shares. In addition, neither we nor any of the underwriters make any representation that the representatives or the lead managers will engage in these transactions or that these transactions, once commenced, will not be discontinued without notice.

Passive Market Making

In connection with this offering, underwriters and selling group members may engage in passive market making transactions in the Series A Non-Cumulative Preferred Shares on the NYSE in accordance with Rule 103 of Regulation M under the Exchange Act during a period before the commencement of offers or sales of Series A Non-Cumulative Preferred Shares and extending through the completion of distribution. A passive market maker must display its bid at a price not in excess of the highest independent bid of that security. However, if all independent bids are lowered below the passive market maker's bid, that bid must then be lowered when specified purchase limits are exceeded.

Electronic Posting of Prospectus Supplement

This prospectus supplement and the accompanying prospectus in electronic format is available on the Internet web site maintained by Merrill Lynch and may be made available on web sites maintained by one or more of the other underwriters or selling group members, if any, participating in this offering. In addition, one or more of the underwriters participating in this offering may distribute this prospectus supplement and the accompanying prospectus electronically. Other than the prospectus supplement and the accompanying prospectus in electronic format, the information on the Internet web sites maintained by the underwriters and selling group members, if any, or any of their affiliates is not a part of this prospectus supplement or the accompanying prospectus. The representatives may agree to allocate a number of Series A Non-Cumulative Preferred Shares to underwriters for sale to their online brokerage account holders. Internet distributions will be allocated by the underwriters and selling group members, if any, that will make Internet distributions on the same basis as other allocations. Merrill Lynch will be facilitating Internet distribution for this offering to certain of its Internet subscription customers.

Delivery of Shares

We currently expect to deliver Series A Non-Cumulative Preferred Shares offered hereby, in book entry form only, through the facilities of The Depositary Trust Company against payment on or about the date which will be the fifth business day following the date of the pricing of the Series A Non-Cumulative Preferred Shares offered hereby ("T+5"). Under Rule 15c6-1 of the Exchange Act,

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trades in the secondary market generally are required to settle in three business days, unless the parties to a trade expressly agree otherwise. Accordingly, purchasers who wish to trade the Series A Non-Cumulative Preferred Shares on the date of pricing or another day which is more than three business days prior to settlement will be required to specify alternative settlement arrangements to prevent a failed settlement.

Other Relationships

Some of the underwriters and their affiliates have engaged in, and may in the future engage in, investment banking and other commercial dealings in the ordinary course of business with us and our affiliates. They have received customary fees and commissions for these transactions.

Certain affiliates of Merrill Lynch own in the aggregate 85,744 of our common shares, according to a Form 13F filed by Merrill Lynch & Co., Inc. on November 7, 2005, on behalf of its operating subsidiaries which are investment managers.

Affiliates of J.P. Morgan Securities Inc., Citigroup Global Markets Inc. and Wachovia Capital Markets, LLC, respectively, are lenders and/or agents under our unsecured revolving loan and letter of credit facility and secured letter of credit facility, dated November 29, 2005.

JPMorgan Chase Bank, N.A., the trustee under the indenture under which ACGL's 7.35% Senior Notes due 2034 were issued, is an affiliate of J.P. Morgan Securities Inc.

An affiliate of Goldman, Sachs & Co. is a minority equity investor in Flatiron Re, a newly formed Bermuda reinsurance company with whom we recently entered into a quota share reinsurance treaty (see "Prospectus Supplement Summary—Recent Developments—Flatiron Retrocession Facility").

Selling Restrictions

Each underwriter has represented, warranted and agreed that: (1) it has not offered or sold and, prior to the expiry of a period of six months from the closing date, will not offer or sell any Series A Non-Cumulative Preferred Shares to persons in the United Kingdom except to persons whose ordinary activities involve them in acquiring, holding, managing or disposing of investments (as principal or agent) for the purposes of their businesses or otherwise in circumstances which have not resulted and will not result in an offer to the public in the United Kingdom within the meaning of section 102B of the Financial Services and Markets Act of 2000 ("FSMA"); (ii) it has only communicated or caused to be communicated and will only communicate or cause to be communicated any invitation or inducement to engage in investment activity (within the meaning of section 21 of the FSMA) received by it in connection with the issue or sale of any Series A Non-Cumulative Preferred Shares in circumstances in which section 21(i) of the FSMA does not apply to the issuer; and (iii) it has complied and will comply with all applicable provisions of the FSMA with respect to anything done by it in relation to the Series A Non-Cumulative Preferred Shares in, from or otherwise involving the United Kingdom.

In relation to each Member State of the European Economic Area which has implemented the Prospectus Directive (each, a "Relevant Member State"), each underwriter has represented and agreed that with effect from and including the date on which the Prospectus Directive is implemented in that Relevant Member State (the "Relevant Implementation Date") it has not made and will not make an offer of Series A Non-Cumulative Preferred Shares to the public in that Relevant Member State prior to the publication of a prospectus in relation to the Series A Non-Cumulative Preferred Shares which has been approved by the competent authority in that Relevant Member State or, where appropriate, approved in another Relevant Member State and notified to the competent authority in that Relevant Member State, all in accordance with the Prospectus Directive, except that it may, with effect from and including the Relevant Implementation Date, make an offer of Series A Non-Cumulative Preferred

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Shares to the public in that Relevant Member State at any time: (a) to legal entities which are authorized or regulated to operate in the financial markets or, if not so authorized or regulated, whose corporate purpose is solely to invest in securities, (b) to any legal entity which has two or more of (1) an average of at least 250 employees during the last financial year; (2) a total balance sheet of more than €43,000,000 and (3) an annual net turnover of more than €50,000,000, as shown in its last annual or consolidated accounts; or (c) in any other circumstances which do not require the publication by the Company of a prospectus pursuant to Article 3 of the Prospectus Directive.

For the purposes of this provision, the expression an "offer of Series A Non-Cumulative Preferred Shares to the public" in relation to any Series A Non-Cumulative Preferred Shares in any Relevant Member State means the communication in any form and by any means of sufficient information on the terms of the offer and the Series A Non-Cumulative Preferred Shares to be offered so as to enable an investor to decide to purchase or subscribe the Series A Non-Cumulative Preferred Shares, as the same may be varied in that Relevant Member State by any measure implementing the Prospectus Directive in that Relevant Member State and the expression Prospectus Directive means Directive 2003/71/EC and includes any relevant implementing measure in each Relevant Member State.

No underwriter has offered or sold, and will not offer or sell, in Hong Kong, by means of any document, any Series A Non-Cumulative Preferred Shares other than to persons whose ordinary business is to buy or sell shares or debentures, whether as principal or agent, or in circumstances that do not constitute an offer to the public within the meaning of the Companies Ordinance (Cap. 32) of Hong Kong, and unless permitted to do so under the securities laws of Hong Kong, no syndicate member has issued or had in its possession for the purposes of issue, and will not issue or have in its possession for the purpose of issue, any advertisement, document or invitation relating to the Series A Non-Cumulative Preferred Shares other than with respect to the Series A Non-Cumulative Preferred Shares intended to be disposed of to persons outside Hong Kong or only to persons whose business involves the acquisition, disposal or holding of securities whether as principal or agent.

This prospectus supplement and the accompanying prospectus have not been registered as a prospectus with the Monetary Authority of Singapore. Accordingly, this prospectus supplement, the accompanying prospectus, and any other document or material in connection with the offer or sale, or invitation for subscription or purchase, of the Series A Non-Cumulative Preferred Shares may not be circulated or distributed, nor may the Series A Non-Cumulative Preferred Shares be offered or sold, or be made the subject of an invitation for subscription or purchase, whether directly or indirectly, to persons in Singapore other than (i) to an institutional investor under Section 274 of the Securities and Futures Act, Chapter 289 of Singapore (the "SFA"), (ii) to a relevant person, or any person pursuant to Section 275(1A), and in accordance with the conditions, specified in Section 275 of the SFA or (iii) otherwise pursuant to, and in ac-cordance with the conditions of, any other applicable provision of the SFA.

Where the Series A Non-Cumulative Preferred Shares are subscribed or purchased under Section 275 by a relevant person which is: (a) a corporation (which is not an accredited investor) the sole business of which is to hold investments and the entire share capital of which is owned by one or more individuals, each of whom is an accredited investor; or (b) a trust (where the trustee is not an accredited investor) whose sole purpose is to hold investments and each beneficiary is an accredited investor, shares, debentures and units of shares and debentures of that corporation or the beneficiaries' rights and interest in that trust shall not be transferable for 6 months after that corporation or that trust has acquired the shares under Section 275 except: (1) to an institutional investor under Section 274 of the SFA or to a relevant person, or any person pursuant to Section 275(1A), and in accordance with the conditions, specified in Section 275 of the SFA; (2) where no consideration is given for the transfer; or (3) by operation of law.

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The Series A Non-Cumulative Preferred Shares have not been and will not be registered under the Securities and Exchange Law of Japan (the Securities and Exchange Law) and each underwriter has agreed that it will not offer or sell any Series A Non-Cumulative Preferred Shares, directly or indirectly, in Japan or to, or for the benefit of, any resident of Japan (which term as used herein means any person resident in Japan, including any corporation or other entity organized under the laws of Japan), or to others for re-offering or resale, directly or indirectly, in Japan or to a resident of Japan, except pursuant to an exemption from the registration requirements of, and otherwise in compliance with, the Securities and Exchange Law and any other applicable laws, regulations and ministerial guidelines of Japan.

LEGAL MATTERS

Certain legal matters will be passed upon for us by Cahill Gordon & Reindel LLP, New York, New York. The validity of the Series A Non-Cumulative Preferred Shares will be passed upon for us by Conyers Dill & Pearman, Hamilton, Bermuda. Certain legal matters will be passed upon for the underwriters by Willkie Farr & Gallagher LLP, New York, New York.

EXPERTS

The consolidated financial statements of Arch Capital Group Ltd. as of December 31, 2004 and 2003 and for each of the three years in the period ended December 31, 2004, incorporated herein by reference have been audited by PricewaterhouseCoopers LLP, independent accountants, as stated in their report thereon.

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CAUTIONARY NOTE REGARDING FORWARD-LOOKING STATEMENTS

The Private Securities Litigation Reform Act of 1995 provides a "safe harbor" for forward—looking statements. This prospectus supplement, including the documents incorporated herein by reference, or any other written or oral statements made by or on behalf of us may include forward-looking statements, which reflect our current views with respect to future events and financial performance. All statements other than statements of historical fact included in or incorporated by reference in this prospectus supplement are forward-looking statements. Forward-looking statements can generally be identified by the use of forward-looking terminology such as "may", "will", "expect", "intend", "estimate", "anticipate", "believe" or "continue" or their negative or variations or similar terminology.

Forward-looking statements involve our current assessment of risks and uncertainties. Actual events and results may differ materially from those expressed or implied in these statements. Important factors that could cause actual events or results to differ materially from those indicated in such statements are discussed below and elsewhere in this prospectus supplement, including, without limitation, the section entitled "Risk Factors," and in our periodic reports filed with the Securities and Exchange Commission and include:

•: our ability to successfully implement our business strategy during "soft" as well as "hard" markets;
•: acceptance of our business strategy, security and financial condition by rating agencies and regulators, as well as by brokers and our insureds and reinsureds;
•: our ability to maintain or improve our ratings, which may be affected by our ability to raise additional equity or debt financings, as well as other factors described herein;
•: general economic and market conditions (including inflation, interest rates and foreign currency exchange rates) and conditions specific to the reinsurance and insurance markets in which we operate;
•: competition, including increased competition, on the basis of pricing, capacity, coverage terms or other factors;
•: our ability to successfully integrate establish and maintain operating procedures (including the implementation of improved computerized systems and programs to replace and support manual systems) to effectively support our underwriting initiatives and to develop accurate actuarial data, especially in the light of the rapid growth of our business;
•: the loss of key personnel;
•: the integration of businesses we have acquired or may acquire into our existing operations;
•: accuracy of those estimates and judgments utilized in the preparation of our financial statements, including those related to revenue recognition, insurance and other reserves, reinsurance recoverables, investment valuations, intangible assets, bad debts, income taxes, contingencies, litigation and any determination to use the deposit method of accounting, which, for a relatively new insurance and reinsurance company, like our company, are even more difficult to make than those made in a mature company since limited historical information has been reported to us through September 30, 2005;
•: greater than expected loss ratios on business written by us and adverse development on claim and/or claim expense liabilities related to business written by our insurance and reinsurance subsidiaries;
•: severity and/or frequency of losses;

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•: claims for natural or man-made catastrophic events in our insurance or reinsurance business could cause large losses and substantial volatility in our results of operations;
•: acts of terrorism, political unrest and other hostilities or other unforecasted and unpredictable events;
•: losses relating to aviation business and business produced by a certain managing underwriting agency for which we may be liable to the purchaser of our prior reinsurance business or to others in connection with the May 5, 2000 asset sale described in our periodic reports filed with the SEC;
•: availability to us of reinsurance to manage our gross and net exposures and the cost of such reinsurance;
•: the failure of reinsurers, managing general agents or others to meet their obligations to us;
•: the timing of loss payments being faster or the receipt of reinsurance recoverables being slower than anticipated by us;
•: changes in accounting principles or the application of such principles by accounting firms or regulators;
•: statutory or regulatory developments, including as to tax policy and matters and insurance and other regulatory matters (such as the adoption of proposed legislation that would affect Bermuda-headquartered companies and/or Bermuda-based insurers or reinsurers); and
•: rating agency policies and practices.

In addition, other general factors could affect our results, including: (a) developments in the world's financial and capital markets and our access to such markets; (b) changes in regulations or tax laws applicable to us, our subsidiaries, brokers or customers; and (c) the effects of business disruption or economic contraction due to terrorism or other hostilities.

All subsequent written and oral forward-looking statements attributable to us or persons acting on our behalf are expressly qualified in their entirety by these cautionary statements. The foregoing review of important factors should not be construed as exhaustive and should be read in conjunction with other cautionary statements that are included herein or elsewhere. We undertake no obligation to publicly update or revise any forward-looking statement, whether as a result of new information, future events or otherwise.

WHERE YOU CAN FIND ADDITIONAL INFORMATION

As required by law, we file reports, proxy statements and other information with the SEC. These reports, proxy statements and other information contain additional information about us. You can inspect and copy these materials at the SEC's Public Reference Room at 100 F Street, N.E., Washington, D.C. 20549. You can obtain information about the Public Reference Room by calling the SEC at 1-800-SEC-0330. The SEC maintains an Internet website that contains reports, proxy and information statements and other information regarding companies that file electronically with the SEC. The SEC's Internet address is http: //www.sec.gov. You can also inspect these materials at the offices of the NASDAQ Stock Market, 1735 K Street, N.W., Washington, D.C. 20006.

The SEC allows us to "incorporate by reference" information into this prospectus supplement in addition to the information incorporated by reference in the accompanying prospectus, which means that we can disclose important information by referring you to another document filed separately with the SEC. This information incorporated by reference is considered part of this prospectus supplement.

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except to the extent that the information is superseded by information in this prospectus supplement. This prospectus supplement incorporates by reference

•: our Annual Report on Form 10-K for the year ended December 31, 2004;
•: our Definitive Proxy Statement on Schedule 14A, filed on March 31, 2005;
•: our Quarterly Reports on Form 10-Q for the quarters ended March 31, 2005, June 30, 2005 and September 30, 2005; and
•: our Current Reports on Form 8-K filed with the SEC on January 26, 2006, January 23, 2006, January 4, 2006, December 7, 2005, November 14, 2005, November 16, 2005, November 29, 2005 and November 30, 2005.

We are also incorporating by reference the information contained in all other documents that we file with the SEC between the date of this prospectus supplement and the earlier of the termination of this offering and the time that all of the Series A Non-Cumulative Preferred Shares offered hereby are sold. The information contained in any of these documents will be considered part of this prospectus supplement from the date these documents are filed.

Any statement contained in this prospectus supplement or in a document incorporated or deemed to be incorporated by reference herein will be deemed to be modified or superseded for purposes of this prospectus supplement to the extent that a statement contained herein or in any other subsequently filed document which also is or is deemed to be incorporated by reference herein modifies or supersedes such statement. Any statement so modified or superseded will not be deemed, except as so modified or superseded, to constitute a part of this prospectus supplement.

If you would like to receive a copy of any document incorporated by reference into this prospectus supplement (which will not include any of the exhibits to the document other than those exhibits that are themselves specifically incorporated by reference into this prospectus supplement), you should call or write to Arch Capital Group Ltd., Clarendon House, 2 Church Street, Hamilton HM 11, Bermuda, Attention: Secretary (telephone (441) 295-1422). We will provide these documents, without charge, by first class mail.

Our common shares are quoted on the NASDAQ National Market. We intend to apply to have the Series A Non-Cumulative Preferred Shares approved for listing on the NYSE.

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$650,000,000

ARCH CAPITAL GROUP LTD.

Common Shares, Preference Shares, Unsecured Debt Securities, Depositary Shares, Warrants to Purchase Common Shares, Preference Shares or Debt Securities, Share Purchase Contracts and Share Purchase Units

ARCH CAPITAL GROUP (U.S.) INC.

Senior Debt Securities Fully and Unconditionally Guaranteed by Arch Capital Group Ltd.

The following are types of securities that may be offered and sold under this prospectus up to an aggregate initial offering price of $650,000,000:

•: Arch Capital Group Ltd. Common shares
•: Arch Capital Group Ltd. Preference shares
•: Arch Capital Group Ltd. Unsecured debt securities
•: Arch Capital Group Ltd. Depositary shares representing preference shares or common shares
•: Arch Capital Group Ltd. Warrants to purchase common shares, preference shares or debt securities
•: Arch Capital Group Ltd. Share purchase contracts and share purchase units
•: Arch Capital Group (U.S.) Inc. Senior debt securities

A prospectus supplement, which must accompany this prospectus, will describe the securities Arch Capital Group Ltd. and/or Arch Capital Group (U.S.) Inc. are offering and selling, as well as the specific terms of the securities. Those terms may include, among others, as applicable:

• Aggregate principal amount		• Sinking fund terms
• Issue price		• Ranking
• Denomination		• Redemption terms
• Currency or composite currency		• Conversion terms
• Maturity		• Listing on a securities exchange
• Interest rate		• Amount payable at maturity
• Dividend rate		• Liquidation preference

The prospectus supplement may also supplement or update information contained in this prospectus; provided, that such information does not constitute material changes to the information herein such that it alters the nature of the offering or the securities offered.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

Arch Capital Group Ltd. common shares are traded on the Nasdaq National Market under the symbol "ACGL". The closing price of Arch Capital Group Ltd. common shares was $40.00 per share on July 1, 2004.

INVESTING IN OUR SECURITIES INVOLVES CERTAIN RISKS. SEE "RISK FACTORS" BEGINNING ON PAGE 2.

August 25, 2004

TABLE OF CONTENTS

		Page
ARCH CAPITAL GROUP LTD		1
ARCH CAPITAL GROUP (U.S.) INC		1
RISK FACTORS		2
RATIO OF EARNINGS TO FIXED CHARGES AND PREFERENCE SHARE DIVIDENDS		20
USE OF PROCEEDS		21
GENERAL DESCRIPTION OF THE OFFERED SECURITIES		21
DESCRIPTION OF ARCH CAPITAL SHARE CAPITAL		21
DESCRIPTION OF ARCH CAPITAL COMMON SHARES		22
DESCRIPTION OF ARCH CAPITAL PREFERENCE SHARES		23
DESCRIPTION OF ARCH CAPITAL DEBT SECURITIES		26
DESCRIPTION OF ARCH CAPITAL DEPOSITARY SHARES		40
DESCRIPTION OF ARCH CAPITAL WARRANTS TO PURCHASE COMMON SHARES OR PREFERENCE SHARES		43
DESCRIPTION OF ARCH CAPITAL WARRANTS TO PURCHASE DEBT SECURITIES		45
DESCRIPTION OF ARCH CAPITAL SHARE PURCHASE CONTRACTS AND SHARE PURCHASE UNITS		46
DESCRIPTION OF ARCH CAPITAL GROUP (U.S.) INC. SENIOR DEBT SECURITIES		47
PLAN OF DISTRIBUTION		60
WHERE YOU CAN FIND MORE INFORMATION		62
INCORPORATION OF DOCUMENTS BY REFERENCE		63
ABOUT THIS PROSPECTUS		64
CAUTIONARY NOTE REGARDING FORWARD-LOOKING STATEMENTS		64
LEGAL MATTERS		66
EXPERTS		66
ENFORCEMENT OF CIVIL LIABILITIES UNDER UNITED STATES FEDERAL SECURITIES LAWS		67

We have obtained consent from the Bermuda Monetary Authority for the issue and transfer of shares to and between persons regarded as non-resident in Bermuda for exchange control purposes. Issues and transfers of shares to any person regarded as resident in Bermuda for exchange control purposes may require specific prior approval from the Bermuda Monetary Authority. The Bermuda Monetary Authority accepts no responsibility for the financial soundness of any proposal or for the correctness of any of the statements made or opinions expressed in this prospectus.

ARCH CAPITAL GROUP LTD.

Arch Capital Group Ltd., a Bermuda-based company, provides insurance and reinsurance on a worldwide basis through its wholly owned subsidiaries. While Arch Capital is positioned to provide a full range of property and casualty insurance and reinsurance lines, it focuses on writing specialty lines of insurance and reinsurance.

Our principal executive office is located at Wessex House, 45 Reid Street, Hamilton HM 12 Bermuda (telephone number: (441) 278-9250). Additional information about Arch Capital may be found over the Internet at our website at http://www.archcapgroup.bm. The information on our website is not a part of this prospectus and is not incorporated by reference herein.

You can obtain additional information about us in the reports and other documents incorporated by reference in this prospectus. See "Where You Can Find More Information" and "Incorporation of Documents by Reference."

ARCH CAPITAL GROUP (U.S.) INC.

Arch Capital Group (U.S.) Inc. is a Delaware corporation. Arch US is a wholly owned subsidiary of Arch Capital. Arch US is a holding company for the U.S.-based insurance and reinsurance operations of Arch Capital. Its principal executive office is located at 20 Horseneck Lane, Greenwich, Connecticut (telephone number: (203) 862-4310).

RISK FACTORS

Before you invest in our securities, you should carefully consider the risks involved. Accordingly, you should carefully consider these risk factors, as well as the information under the heading "Cautionary Note Regarding Forward-Looking Statements," the information contained in or incorporated by reference into this prospectus and information contained in or incorporated by reference into any prospectus supplements relating to specific offerings of securities.

Risks Relating to Our Industry

We operate in a highly competitive environment, and since the September 11, 2001 events, new capital has entered the market; these factors may mitigate the benefits that the financial markets may perceive for the property and casualty insurance and reinsurance industry, and we may not be able to compete successfully in our industry.

The insurance and reinsurance industry is highly competitive. We compete with major U.S. and non-U.S. insurers and reinsurers, many of which have greater financial, marketing and management resources than we do. We also compete with new companies that continue to be formed to enter the insurance and reinsurance markets. In our insurance business, we compete with insurers that provide property and casualty lines of insurance, including ACE Limited, Allied World Assurance Company, Ltd., American International Group, Inc., AXIS Capital Holdings Limited, Berkshire Hathaway, Inc., Chubb Corporation, Converium Group, Endurance Specialty Holdings Ltd., The Hartford Financial Services Group, Inc., Lloyd's of London, The St. Paul Travelers Companies and XL Capital Ltd. In our reinsurance business, we compete with reinsurers that provide property and casualty lines of reinsurance, including ACE Limited, AXIS Capital Holdings Limited, Converium Group, Endurance Specialty Holdings Ltd., Everest Re Group Ltd., General Reinsurance Corporation, Hannover Rückversicherung AG, Lloyd's of London, Montpelier Re Holdings Ltd., Munich Re Group, PartnerRe Ltd., Platinum Underwriters Holdings, Ltd., RenaissanceRe Holdings Ltd., Swiss Reinsurance Company, Transatlantic Holdings, Inc. and XL Capital Ltd. We do not believe that we have a significant market share in any of our markets.

Trends toward consolidation in the insurance industry could also lead to pricing pressure and lower margins for insurers and reinsurers. In addition, since the events of September 11, 2001, several newly formed offshore entities have entered the market to address the capacity issues in the insurance and reinsurance industry. Several publicly traded insurance and reinsurance companies have also raised additional capital to meet perceived demand in the current environment. Since September 11, 2001, newly formed and existing insurance industry companies have reportedly raised additional capital, and some industries (in particular, the airline industry) have announced that they may form industry consortia to provide insurance coverage for their members, thereby taking those lines out of the commercial insurance and reinsurance markets in which we operate. Financial institutions and other capital markets participants also offer alternative products and services similar to our own or alternative products that compete with insurance and reinsurance products. In addition, we may not be aware of other companies that may be planning to enter the segments of the insurance and reinsurance market in which we operate or of existing companies that may be planning to raise additional capital.

Our competitive position is based on many factors, including our perceived overall financial strength, ratings assigned by independent rating agencies, geographic scope of business, client relationships, premiums charged, contract terms and conditions, products and services offered (including the ability to design customized programs), speed of claims payment, reputation, experience and qualifications of employees and local presence. We may not be successful in competing with others in our industry on any of these bases, and the intensity of competition in our industry may erode profitability for insurance and reinsurance companies generally, including us. In addition, we may not be able to participate at all or to the same extent as more established or other companies in any price

increases or increased profitability in our industry. If we do not share in such price increases or increased profitability, our financial condition and results of operations could be materially adversely affected.

The insurance and reinsurance industry is highly cyclical, and we expect to experience periods characterized by excess underwriting capacity and unfavorable premium rates.

Although we believe that we do not have exposure to the events of September 11, 2001 because we did not have insurance in force at that time with respect to exposure to such events, we now have substantial exposure to unexpected, large losses resulting from future man-made catastrophic events, such as acts of war, acts of terrorism and political instability. These risks are inherently unpredictable, although recent events may lead to increased frequency and severity of losses. It is difficult to predict the timing of such events with statistical certainty or estimate the amount of loss any given occurrence will generate. In certain instances, we specifically insure and reinsure risks resulting from acts of terrorism. Even in cases where we attempt to exclude losses from terrorism and certain other similar risks from some coverages written by us, we may not be successful in doing so. Moreover, irrespective of the clarity and inclusiveness of policy language, a result-oriented court or arbitration panel favoring the insured or ceding company may choose not to enforce the language as written; such a tribunal may adopt a strained interpretation of the policy language, invoke public policy to limit enforceability of policy language, ignore policy language, make factual findings unwarranted by the evidence or otherwise seek to justify a ruling adverse to us. Accordingly, while we believe our reinsurance programs, together with the coverage provided under the Terrorism Risk Insurance Act of 2002 ("TRIA"), are sufficient to reasonably limit our net losses relating to potential future terrorist attacks, we can offer no assurance that our reserves will be adequate to cover losses when they materialize. To the extent that an act of terrorism is certified by the Secretary of the Treasury, we may be covered under TRIA for up to 90% of our losses, subject to certain mandatory deductibles. It is not possible to eliminate completely our exposure to unforecasted or unpredictable events, and to the extent that losses from such risks occur, our financial condition and results of operations could be materially adversely affected.

Claims for catastrophic events could cause large losses and substantial volatility in our results of operations, and, as a result, the value of our common shares may fluctuate widely.

Although we have not experienced significant losses resulting from catastrophic events since the commencement of our underwriting initiative in October 2001, we have large aggregate exposures to natural disasters. Catastrophes can be caused by various events, including hurricanes, floods, windstorms, earthquakes, hailstorms, explosions, severe winter weather and fires. Catastrophes can also cause losses in non-property business such as workers' compensation or general liability. In addition to the nature of the property business, we believe that economic and geographic trends affecting insured property, including inflation, property value appreciation and geographic concentration tend to generally increase the size of losses from catastrophic events over time. Therefore, claims for catastrophic events could expose us to large losses and cause substantial volatility in our results of operations, which could cause the value of our common shares to fluctuate widely.

Underwriting claims and reserving for losses are based on probabilities and related modeling, which are subject to inherent uncertainties.

As of March 31, 2004, our reserves for unpaid losses and loss adjustment expenses, net of unpaid losses and loss adjustment expenses recoverable, were approximately $1.9 billion. Such reserves were established in accordance with applicable insurance laws and generally accepted accounting principles ("GAAP"). Although we believe we have applied a conservative reserving philosophy for both our insurance and reinsurance operations, insurance loss reserves are inherently subject to uncertainty. In establishing the reserves for losses and loss adjustment expenses, we have made various assumptions relating to the pricing of our reinsurance contracts and insurance policies and have also considered available historical industry experience and current industry conditions. Our reserving process reflects that there is a possibility that the assumptions made could prove to be inaccurate due to several factors, including the fact that very limited historical information has been reported to us through March 31, 2004.

The failure of any of the loss limitation methods we employ could have a material adverse effect on our financial condition or results of operations.

proportional treaties, we generally seek per occurrence limitations or loss ratio caps to limit the impact of losses from any one event. We cannot be sure that any of these loss limitation methods will be effective. We also seek to limit our loss exposure by geographic diversification. Geographic zone limitations involve significant underwriting judgments, including the determination of the area of the zones and the inclusion of a particular policy within a particular zone's limits. There can be no assurance that various provisions of our policies, such as limitations or exclusions from coverage or choice of forum, will be enforceable in the manner we intend. Disputes relating to coverage and choice of legal forum may also arise. Underwriting is inherently a matter of judgment, involving important assumptions about matters that are inherently unpredictable and beyond our control, and for which historical experience and probability analysis may not provide sufficient guidance. One or more catastrophic or other events could result in claims that substantially exceed our expectations, which could have a material adverse effect on our financial condition or our results of operations, possibly to the extent of eliminating our shareholders' equity and could have a material adverse effect on our ability to make payments on our debt securities.

For our natural catastrophe exposed business, we seek to limit the amount of exposure we will assume from any one insured or reinsured and the amount of the exposure to catastrophe losses in any geographic zone. We monitor our exposure to catastrophic events, including earthquake and wind, and periodically reevaluate the estimated probable maximum pre-tax loss for such exposures. Our estimated probable maximum pre-tax loss is determined through the use of modeling techniques, but such estimate does not represent our total potential loss for such exposures. We seek to limit the probable maximum pre-tax loss to a specific level for severe catastrophic events. Currently, we generally seek to limit the probable maximum pre-tax loss to approximately 25% of total shareholders' equity for a severe catastrophic event in any geographic zone that could be expected to occur once in every 250 years. There can be no assurances that we will not suffer pre-tax losses greater than 25% of total shareholders' equity from one or more catastrophic events due to several factors, including the inherent uncertainties in estimating the frequency and severity of such events and the margin of error in making such determinations resulting from potential inaccuracies and inadequacies in the data provided by clients and brokers, the modeling techniques and the application of such techniques. In addition, depending on business opportunities and the mix of business that may comprise our insurance and reinsurance portfolio, we may seek to adjust our self-imposed limitations on probable maximum pre-tax loss for catastrophe exposed business.

For the purposes of limiting our risk of loss, we use reinsurance and also may use retrocessional arrangements. In the normal course of business, our insurance subsidiaries cede a substantial portion of their premiums to unaffiliated entities. Our reinsurance subsidiaries are currently retaining substantially all of their assumed reinsur-ance premiums written. For the three months ended March 31, 2004, ceded premiums written represented approximately 12.5% of gross premiums written, compared to 15.1% for the year ended December 31, 2003.

The availability and cost of reinsurance and retrocessional protection is subject to market conditions, which are beyond our control. Currently, the market for these arrangements is experiencing high demand for various products resulting in significant rate increases and substantial improvements in terms and conditions since the events of September 11, 2001. Although we believe that our insurance subsidiaries have been successful in obtaining reinsurance protection since the commencement of our underwriting initiative in October 2001, it is not certain that we will be able to continue to obtain adequate protection at cost effective levels. As a result of such market conditions and other factors, we may not be able successfully to mitigate risk through reinsurance and retrocessional arrangements.

Further, we are subject to credit risk with respect to our reinsurance and retrocessions because the ceding of risk to reinsurers and retrocessionaires does not relieve us of our liability to the clients or companies we insure or reinsure. Our failure to establish adequate reinsurance or retrocessional arrangements or the failure of our existing reinsurance or retrocessional arrangements to protect us from overly concentrated risk exposure could adversely affect our financial condition and results of operations. We monitor the financial condition of our reinsurers and attempt to place coverages only with carriers we view as substantial and financially sound. At March 31, 2004, approximately 81.2% of our reinsurance recoverables on paid and unpaid losses of $486.1 million (not including prepaid reinsurance premiums) were due from carriers which had an A.M. Best rating of "A-" or better. Our recoverables on paid and unpaid losses from Sentry Insurance a Mutual Company ("Sentry") represented 4.7% of our total shareholders' equity at March 31, 2004. No other reinsurance recoverables exceeded 3.4% of our total shareholders' equity at such date. In connection with our acquisition of Arch Specialty in February 2002, the seller, Sentry, agreed to assume all liabilities arising out of Arch Specialty's business prior to the closing of the acquisition. In addition to the guarantee provided by Sentry, substantially all of the recoverable from Sentry is still subject to the original rein-surance agreements inuring to Arch Specialty and, to the extent Sentry fails to comply with its payment obligations to us, we may obtain reimbursement from the third party reinsurers under such agreements.

Our reliance on brokers subjects us to their credit risk.

In accordance with industry practice, we generally pay amounts owed on claims under our insurance and reinsurance contracts to brokers, and these brokers, in turn, pay these amounts to the clients that have purchased insurance or reinsurance from us. In some jurisdictions, if a broker fails to make such payment, we may remain liable to the insured or ceding insurer for the deficiency. Likewise, in certain jurisdictions, when the insured or ceding company pays premium for these contracts to brokers for payment to us, these premiums are considered to have been paid and the insured or ceding company will no longer be liable to us for those amounts, whether or not we have actually received the premiums from the broker. Consequently, we assume a degree of credit risk associated with our brokers. To date, we have not experienced any losses related to this credit risk.

As a result of recent events and instability in the marketplace for insurance products, there is the potential for government intervention in our industry which could hinder our flexibility and negatively affect the business opportunities we perceive are available to us in the market.

In response to the current tightening of supply in certain insurance markets, as well as the impact of the September 11, 2001 events, it is possible that the United States and other governments worldwide may intervene in the insurance and reinsurance markets. Government regulators are generally concerned with the protection of policyholders to the exclusion of other constituencies, including shareholders. While we cannot predict the type of government intervention that may occur or its timing, such intervention could materially adversely affect us by:

•: disproportionately benefiting one country's companies over companies in other countries;

•: providing insurance and reinsurance capacity in the markets and to the customers that we target;
•: regulating the terms of insurance and reinsurance policies; or
•: mandating participation in guaranty associations or other involuntary industry pools.

For example, on November 26, 2002, President Bush signed TRIA into law, which established a federal backstop for insurance-related losses resulting from any act of terrorism carried out by foreign powers on U.S. soil or against U.S. air carriers, vessels or foreign missions. Under TRIA, all U.S.-based property and casualty insurers are required to make terrorism insurance coverage available in specified commercial property and casualty insurance lines. In return, TRIA provides that the federal government will pay 90% of covered losses after an insurer's losses exceed a deductible determined by a statutorily prescribed formula, up to a combined annual aggregate limit for the federal government and all insurers of $100 billion. If an act (or acts) of terrorism result in covered losses exceeding the $100 billion annual limit, insurers with losses exceeding their deductibles will not be responsible for additional losses. The deductible for each year is based on the insurer's direct commercial earned premiums for property and casualty insurance for the prior calendar year multiplied by a specified percentage. The specified percentages are 7% for 2003, 10% for 2004 and 15% for 2005, respectively.

Our U.S.-based property and casualty insurers, Arch Insurance, Arch Specialty, Arch E&S and Western Diversified, are subject to TRIA. TRIA specifically excludes reinsurance and personal lines business and, accordingly, currently does not apply to our non-standard automobile business or our reinsurance operations. Based on 2003 direct commercial earned premiums, our U.S. insurance group's deductible for 2004 would be $104.1 million (i.e., 10% of such earned premiums). The amount of our deductible for 2005 could increase substantially, depending upon the amount of direct commercial earned premiums we write in 2004, and in light of the fact that the deductible percentage increases in such years. Currently, there is uncertainty as to what effect TRIA will have on the insurance industry.

The insurance industry is also affected by political, judicial and legal developments which have in the past resulted in new or expanded theories of liability. These or other changes could impose new financial obligations on us by extending coverage beyond our underwriting intent or otherwise, require us to make unplanned modifications to the products and services that we provide, or cause the delay or cancellation of products and services that we provide. In some instances, these changes may not become apparent until some time after we have issued insurance or reinsurance contracts that are affected by the changes. As a result, the full extent of liability under our insurance or reinsurance contracts may not be known for many years after a contract is issued. The effects of unforeseen developments or substantial government intervention could adversely impact our ability to achieve our goals.

Risks Relating to Our Company

Our future performance is difficult to predict because we have a limited operating history.

We began our underwriting initiative in October 2001, and have limited operating and financial history. As a result, there is limited historical financial and operating information available to help potential investors evaluate our performance or an investment in our common shares. Insurance companies in their initial stages of development face substantial business and financial risks and may suffer significant losses. These new companies must successfully develop business relationships, establish operating procedures, hire staff, install management information and other systems and complete other

tasks necessary to conduct their intended business activities. As a result of these risks, it is possible that we will not be successful in implementing our business strategy or accomplishing these necessary tasks. In addition, because we have very limited financial data on which to base our reserves for losses and loss adjustment expenses, our historical financial results may not accurately provide an indication of our future performance.

Our success will depend on our ability to establish and maintain effective operating procedures and internal controls.

As a relatively new insurance and reinsurance company, our success will also be dependent upon our ability to establish and maintain operating procedures and internal controls (including the timely and successful implementation of our information technology initiatives, which include the implementation of improved computerized systems and programs to replace and support manual systems) to effectively support our business and our regulatory and reporting requirements. We may not be successful in such efforts. We have been, and are continuing to, enhance our procedures and controls, including our controls over financial reporting. Our management does not expect that our disclosure controls or our internal controls will prevent all errors and all fraud. A control system, no matter how well conceived and operated, can provide only reasonable, not absolute, assurance that the objectives of the control system are met. Further, the design of a control system must reflect the fact that there are resource constraints, and the benefits of controls must be considered relative to their costs. As a result of the inherent limitations in all control systems, no evaluation of controls can provide absolute assurance that all control issues and instances of fraud, if any, within the company have been detected. These inherent limitations include the realities that judgments in decision-making can be faulty, and that breakdowns can occur because of simple error or mistake. Additionally, controls can be circumvented by the individual acts of some persons or by collusion of two or more people. The design of any system of controls also is based in part upon certain assumptions about the likelihood of future events, and there can be no assurance that any design will succeed in achieving its stated goals under all potential future conditions; over time, controls may become inadequate because of changes in conditions, or the degree of compliance with the policies or procedures may deteriorate. As a result of the inherent limitations in a cost-effective control system, misstatement due to error or fraud may occur and not be detected. Accordingly, our disclosure controls and procedures are designed to provide reasonable, not absolute, assurance that the disclosure controls and procedures are met.

The loss of our key employees or our inability to retain them could negatively impact our business.

The preparation of our financial statements requires us to make many estimates and judgments, which are even more difficult than those made in a mature company since very limited historical information has been reported to us through March 31, 2004.

The preparation of consolidated financial statements requires us to make many estimates and judgments that affect the reported amounts of assets, liabilities (including reserves), revenues and expenses, and related disclosures of contingent liabilities. On an ongoing basis, we evaluate our estimates, including those related to revenue recognition, insurance and other reserves, reinsurance recoverables, investment valuations, intangible assets, bad debts, income taxes, contingencies and litigation. We base our estimates on historical experience, where possible, and on various other assumptions that we believe to be reasonable under the circumstances, which form the basis for our judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Estimates and judgments for a relatively new insurance and reinsurance company, like our company, are even more difficult to make than those made in a mature company since very limited historical information has been reported to us through March 31, 2004. Instead, our current loss reserves are based almost entirely on estimates involving actuarial and statistical projections of our expectations of the ultimate settlement and administration costs of claims incurred but not yet reported. We utilize actuarial models as well as historical insurance industry loss development patterns to establish loss reserves. Accordingly, actual claims and claim expenses paid may deviate, perhaps substantially, from the reserve estimates reflected in our financial statements.

The Warburg Pincus funds and the Hellman & Friedman funds together own a majority of our voting shares, and these shareholders have the right to have directors on our board and the right to approve most transactions outside of the ordinary course of our business; their interests may materially differ from the interests of the holders of our common shares.

The Warburg Pincus funds and the Hellman & Friedman funds own 31.9% and 20.0% of our outstanding voting shares, respectively, as of March 31, 2004. These shareholders are non-U.S. persons as defined in the Internal Revenue Code of 1986, as amended (the "Code"), and, as such, they are not subject to the voting limitation contained in our bye-laws. In addition, our shareholders agreement prevents us from taking many actions outside the ordinary course of our business without the approval of a designee of the Warburg Pincus funds and a designee of the Hellman & Friedman funds. We have agreed not to declare any dividend or make any other distribution on our common shares, and not to repurchase any common shares, until we have repurchased from the Warburg Pincus funds, the Hellman & Friedman funds and the other holders of preference shares, pro rata, on the basis of the amount of these shareholders investments in us at the time of such repurchase, preference shares having an aggregate value of $250.0 million, at a per share price acceptable to these shareholders. By reason of their ownership and the shareholders agreement between us and the holders of preference shares, the Warburg Pincus funds and the Hellman & Friedman funds, individually or together, are able to strongly influence or effectively control actions to be taken by us, or our shareholders.

In addition, the Warburg Pincus funds and the Hellman & Friedman funds are entitled to nominate a prescribed number of directors based on the respective retained percentages of their equity securities purchased in November 2001. Currently, our board consists of twelve members, which includes three directors nominated by the Warburg Pincus funds and two directors nominated by the Hellman & Friedman funds. As long as the Warburg Pincus funds retain at least 75% of their original investment and the Hellman & Friedman funds retain at least 60% of their original investment, these shareholders will be entitled to nominate six and three directors, respectively. Together they have the right to nominate a majority of directors to our board. The interests of these shareholders may differ materially from the interests of the holders of our common shares, and these shareholders could take actions or make decisions that are not in the interests of the holders of our common shares generally.

We may be required to issue additional preference shares to the investors in the November 2001 capital infusion as a result of a purchase price adjustment mechanism agreed to in connection with it, and the value of our common shares may, therefore, be further diluted.

Pursuant to the subscription agreement entered into in connection with the November 2001 capital infusion (the "Subscription Agreement"), an adjustment basket relating to certain non-core operations was calculated during the 2003 fourth quarter for purposes of determining whether we would be required to issue additional preference shares to the investors as a purchase price adjustment. The adjustment basket was equal to (1) the difference between value realized upon sale and the GAAP book value at the closing of the capital infusion (November 2001) (as adjusted based on a pre-determined growth rate) of agreed upon non-core businesses; plus (2) the difference between GAAP net book value of our insurance balances attributable to our core insurance operations with respect to any policy or contract written or having a specified effective date at the time of the final adjustment and those balances at the closing; minus (3) reductions in book value arising from costs and expenses relating to the transaction provided under the Subscription Agreement, actual losses arising out of breach of representations under the Subscription Agreement and certain other costs and expenses. If the adjustment basket had been calculated as less than zero, we would have been required to issue additional preference shares (or, in certain extreme cases, preference shares of a subsidiary) to the investors based on the decrease in value of the components of the adjustment basket. In February 2004, the parties agreed that no purchase price adjustment was required pursuant to the above calculation and, accordingly, no additional preference shares will be issued to the investors. In November 2005, there will be a calculation of a further adjustment basket based on (1) liabilities owed to Folksamerica Reinsurance Company (if any) under the Asset Purchase Agreement, dated as of January 10, 2000, between us and Folksamerica, and (2) specified tax and ERISA matters under the Subscription Agreement.

The price of our common shares may be volatile.

There has been significant volatility in the market for equity securities. In 2001 and 2002, the price of our common shares fluctuated from a low of $14.38 to a high of $28.34 and from a low of $22.85 to a high of $34.50, respectively. For the year ended December 31, 2003, the price of our common shares fluctuated from a low of $27.71 to a high of $40.01, and for the two fiscal quarters ended June 30, 2004 the price of our common shares fluctuated from a low of $36.61 to a high of $45.19. On July 1, 2004, the last quoted price of our common shares as reported on the Nasdaq National Market was $40.00. The price of our common shares may not remain at or exceed current levels. The following factors may have an adverse impact on the market price of our common stock:

•: actual or anticipated variations in our quarterly results of operations;
•: changes in market valuation of companies in the insurance and reinsurance industry;
•: changes in expectations of future financial performance or changes in estimates of securities analysts;
•: fluctuations in stock market process and volumes;
•: issuances or sales of common shares or other securities in the future;
•: the addition or departure of key personnel; and
•: announcements by us or our competitors of acquisitions, investments or strategic alliances.

Stock markets in the United States often experience extreme price and volume fluctuations. Market fluctuations, as well as general political and economic conditions such as recession or interest rate or currency rate fluctuations, could adversely affect the market price of our stock.

Future sales of our common shares, whether by us or our shareholders, could adversely affect their market price.

Generally, our board of directors has the power to issue new equity (to the extent of authorized shares) without shareholder approval, except that shareholder approval may be required under applicable law or NASDAQ National Market rules for certain transactions. We may issue new equity to raise additional capital to support our insurance and reinsurance operations or for other purposes. Any additional issuance by us would have the effect of diluting the percentage ownership of our shareholders and could have the effect of diluting our earnings and our book value per share.

In addition, the market price of our common shares could fall substantially if our existing shareholders sell large amounts of common shares in the public market. The availability of a large number of shares for sale could result in the need for sellers to accept a lower price in order to complete a sale. As of March 31, 2004, there are 33,552,344 common shares outstanding and up to 44,046,427 common shares issuable upon exercise of options or warrants or conversion of convertible securities. Of the outstanding shares, 26,845,957 common shares are freely tradable and 45,071,359 common shares (including common shares issuable upon conversion of convertible preference shares) are subject to Rule 144 under the Securities Act. Of the shares subject to Rule 144 under the Securities Act, there are 11,989,347 common shares registered for resale by selling shareholders, including those registered pursuant to a registration statement on Form S-3 declared effective on January 27, 2004. In addition, we have registered with the SEC up to $650,000,000 of new securities which may consist in part or entirely of common shares.

We have granted the Warburg Pincus funds and Hellman & Friedman funds demand registration rights and all of the investors in the November 2001 capital infusion certain "piggy-back" registration rights with respect to the common shares issuable to them upon conversion of the preference shares or exercise of the class A warrants (although such warrants are no longer outstanding). Certain other investors who purchased or acquired shares in unregistered transactions also have demand and piggy-back registration rights. They can exercise these rights at any time.

We market our insurance and reinsurance products primarily through brokers. We derive a significant portion of our business from a limited number of brokers. During 2003, approximately 20.0%, 18.3% and 10.6% of our gross premiums written were generated by AON Corporation and its subsidiaries, Marsh & McLennan Companies and Willis Group Holdings and its subsidiaries, respectively. Some of our competitors have had longer term relationships with the brokers we use than we have, and the brokers may promote products offered by companies that may offer a larger variety of products than we do. Loss of all or a substantial portion of the business provided by these brokers could have a material adverse effect on us.

underwriting authorities or otherwise breach obligations owed to us. We have experienced breaches by certain of our agents, all of which have been or are being resolved. To the extent that our agents exceed their authorities or otherwise breach obligations owed to us in the future, our financial condition and results of operations could be materially adversely affected.

Financial strength and claims paying ratings from third party rating agencies are instrumental in establishing the competitive positions of companies in our industry. Periodically, rating agencies evaluate us to confirm that we continue to meet their criteria for the ratings assigned to us by them. Rating agencies have been coming under increasing pressure as a result of high-profile corporate bankruptcies and may, as a result, increase their scrutiny of rated companies, revise their rating policies or take other action. Although, since the commencement of our underwriting initiative in October 2001, our ratings have not been downgraded, we can offer no assurances that our ratings will remain at their current levels. A ratings downgrade, or the potential for such a downgrade, could adversely affect both our relationships with agents, brokers, wholesalers and other distributors of our existing products and services and new sales of our products and services. Any ratings downgrade or failure to obtain a necessary rating could adversely affect our ability to compete in our markets and have a material adverse impact on our financial condition and results of operations.

Our investment performance may affect our financial results and ability to conduct business.

Our operating results depend in part on the performance of our investment portfolio. A significant portion of our cash and invested assets consists of fixed income securities (89.4% as of March 31, 2004). Although our current investment guidelines stress preservation of capital, market liquidity and diversification of risk, our investments are subject to market-wide risks and fluctuations. In addition, although we did not experience any significant defaults by issuers during 2003 or to date during 2004, we are subject to risks inherent in particular securities. We may not be able to realize our investment objectives, which could reduce our net income significantly. In the event that we are unsuccessful in correlating our investment portfolio with our expected insurance and reinsurance liabilities, we may be forced to liquidate our investments at times and prices that are not optimal, which could have a material adverse effect on our financial results and ability to conduct our business.

We may be adversely affected by interest rate changes.

Our operating results are affected, in part, by the performance of our investment portfolio. Our investment portfolio contains interest-rate-sensitive instruments, such as bonds, which may be adversely affected by changes in interest rates. Changes in interest rates could also have an adverse effect on our investment income and results of operations. For example, if interest rates decline, as is the case in the current environment, funds reinvested will earn less than expected.

In addition, our investment portfolio includes mortgage-backed securities. As of March 31, 2004, mortgage-backed securities constituted approximately 2.7% of our cash and invested assets. As with other fixed income investments, the fair market value of these securities fluctuates depending on market and other general economic conditions and the interest rate environment. Changes in interest rates can expose us to prepayment risks on these investments. In periods of declining interest rates, mortgage prepayments generally increase and mortgage-backed securities are prepaid more quickly, requiring us to reinvest the proceeds at the then current market rates. During 2003, we significantly decreased our investments in mortgage-backed securities in order to reduce the prepayment risk in our investment portfolio.

Interest rates are highly sensitive to many factors, including governmental monetary policies, domestic and international economic and political conditions and other factors beyond our control. Although we attempt to take measures to manage the risks of investing in a changing interest rate environment, we may not be able to mitigate interest rate sensitivity effectively. Despite our mitigation efforts, a significant increase in interest rates could have a material adverse effect on our book value.

We may require additional capital in the future, which may not be available or only available on unfavorable terms.

On May 5, 2000, we sold our prior reinsurance operations to Folksamerica Reinsurance Company. The Folksamerica transaction was structured as a transfer and assumption agreement (and not reinsurance), and, accordingly, the loss reserves (and any related reinsurance recoverables) relating to the transferred business are not included as assets or liabilities on our balance sheet. In addition, in connection with that asset sale, we made extensive representations and warranties about us and our reinsurance operations, some of which survived the closing of the asset sale. Breach of these representations and warranties could result in liability to us. In the event that Folksamerica refuses or is unable to make payment for reserved losses transferred to it by us in the May 2000 sale and the notice given to reinsureds is found not to be an effective release by such reinsureds, we would be liable for such claims.

Any future acquisitions may expose us to operational risks.

•: integrating financial and operational reporting systems;
•: establishing satisfactory budgetary and other financial controls;
•: funding increased capital needs and overhead expenses;
•: obtaining management personnel required for expanded operations;
•: funding cash flow shortages that may occur if anticipated sales and revenues are not realized or are delayed, whether by general economic or market conditions or unforeseen internal difficulties;

•: the value of assets acquired may be lower than expected or may diminish due to credit defaults or changes in interest rates and liabilities assumed may be greater than expected;
•: the assets and liabilities we may acquire may be subject to foreign currency exchange rate fluctuation; and
•: financial exposures in the event that the sellers of the entities we acquire are unable or unwilling to meet their indemnification, reinsurance and other obligations to us.

Our failure to manage successfully these operational challenges and risks may impact our results of operations.

Some provisions of our bye-laws could have the effect of discouraging unsolicited takeover bids from third parties or changes in management initiated by shareholders. These provisions may encourage companies interested in acquiring the company to negotiate in advance with our board of directors, since the board has the authority to overrule the operation of several of the limitations.

Among other things, our bye-laws provide:

•: for a classified board of directors, in which the directors of the class elected at each annual general meeting holds office for a term of three years, with the term of each class expiring at successive annual general meetings of shareholders;
•: that the number of directors is determined by the board from time to time by a vote of the majority of our board;
•: that directors may only be removed for cause, and cause removal shall be deemed to exist only if the director whose removal is proposed has been convicted of a felony or been found by a court to be liable for gross negligence or misconduct in the performance of his or her duties;
•: that our board has the right to fill vacancies, including vacancies created by an expansion of the board;
•: for limitations on shareholders' right to call special general meetings and to raise proposals or nominate directors at general meetings; and
•: that shareholders may act by written consent only if such consent is unanimous among all shareholders entitled to vote.

The bye-laws also contain a provision limiting the rights of any U.S. person (as defined in Section 7701(a)(30) of the Code) that owns shares of the Company, directly, indirectly or constructively (within the meaning of Section 958 of the Code), representing more than 9.9% of the voting power of all shares entitled to vote generally at an election of directors. The votes conferred by such shares of such U.S. person will be reduced by whatever amount is necessary so that after any such reduction the votes conferred by the shares of such person will constitute 9.9% of the total voting power of all shares entitled to vote generally at an election of directors. Notwithstanding this provision, the board may make such final adjustments to the aggregate number of votes conferred by the shares of any U.S.

person that the board considers fair and reasonable in all circumstances to ensure that such votes represent 9.9% of the aggregate voting power of the votes conferred by all shares of the Company entitled to vote generally at an election of directors. The Company will assume that all shareholders (other than the Warburg Pincus funds and the Hellman & Friedman funds) are U.S. persons unless we receive assurance satisfactory to us that they are not U.S. persons.

•: merger or consolidation of the company into a 10% holder;
•: sale of any or all of our assets to a 10% holder;
•: the issuance of voting securities to a 10% holder; or
•: amendment of these provisions;

In addition, pursuant to the shareholders agreement which we entered into in connection with the November 2001 capital infusion, we cannot engage in transactions outside the ordinary course of our business, including mergers and acquisitions, without the consent of a designee of the Warburg Pincus funds and a designee of the Hellman & Friedman funds. To the extent these provisions discourage takeover attempts, they could deprive our shareholders of opportunities to realize takeover premiums for their shares or could depress the market price of our common shares.

policy forms, and capital adequacy. These agencies are concerned primarily with the protection of policyholders rather than shareholders. Moreover, insurance laws and regulations, among other things:

•: establish solvency requirements, including minimum reserves and capital and surplus requirements;
•: limit the amount of dividends, tax distributions, intercompany loans and other payments our insurance subsidiaries can make without prior regulatory approval;
•: impose restrictions on the amount and type of investments we may hold; and
•: require assessments to pay claims of insolvent insurance companies.

The National Association of Insurance Commissioners, which we call the NAIC, continuously examines existing laws and regulations. We cannot predict the effect that any NAIC recommendations or proposed or future legislation or rule making in the United States or elsewhere may have on our financial condition or operations.

Our Bermuda insurance and reinsurance subsidiary, Arch Re Bermuda, conducts its business from its offices in Bermuda and is not licensed or admitted to do business in any jurisdiction except Bermuda. We do not believe that Arch Re Bermuda is subject to the insurance laws of any state in the United States; however, recent scrutiny of the insurance and reinsurance industry in the U.S. and other countries could subject Arch Re Bermuda to additional regulation. Our U.S. reinsurance subsidiary, Arch Re U.S., and our U.S. insurance subsidiaries, Arch Insurance, Arch Specialty, Arch E&S, Western Diversified, American Independent and PSIC, write reinsurance and insurance in the United States. These subsidiaries are subject to extensive regulation under state statutes which delegate regulatory, supervisory and administrative powers to state insurance commissioners. Such regulation generally is designed to protect policyholders rather than investors.

Each of our U.S. and Bermuda insurance and reinsurance subsidiaries is required to maintain minimum capital and surplus as mandated by their respective jurisdictions of incorporation. All of our subsidiaries are currently in compliance with these capital and surplus requirements.

If ACGL or any of our subsidiaries were to become subject to the laws of a new jurisdiction in which such entity is not presently admitted, ACGL or such subsidiary may not be in compliance with the laws of the new jurisdiction. Any failure to comply with applicable laws could result in the imposition of significant restrictions on our ability to do business, and could also result in fines and other sanctions, any or all of which could adversely affect our financial condition and results of operations.

ACGL is a holding company and is dependent on dividends and other payments from its operating subsidiaries, which are subject to dividend restrictions, to make payments, including any payments at principal, interest or premium on any debt securities.

We are a holding company whose assets primarily consist of the shares in our subsidiaries. Generally, we depend on our available cash resources, liquid investments and dividends or other distributions from our subsidiaries to make payments, including the payment of debt service obligations, including payments of principal or interest or premium, if any, on any debt securities, and operating expenses we may incur. The ability of our regulated insurance and reinsurance subsidiaries to pay dividends or make distributions is dependent on their ability to meet applicable regulatory standards. Since the commencement of our underwriting initiative in October 2001 through March 31, 2004, no

dividends were paid to ACGL by any of its subsidiaries. We believe that we have enough cash resources and available dividend capacity to service our indebtedness and other current outstanding obligations.

The ability of our regulated insurance and reinsurance subsidiaries to pay dividends or make distributions is dependent on their ability to meet applicable regulatory standards. Under Bermuda law, Arch Re Bermuda is required to maintain a minimum solvency margin (i.e., the amount by which the value of its general business assets must exceed its general business liabilities) equal to the greatest of (1) $100,000,000, (2) 50% of net premiums written (being gross premiums written by us less any premiums ceded by us, but we may not deduct more than 25% of gross premiums when computing net premiums written) and (3) 15% of loss and other insurance reserves. Arch Re Bermuda is prohibited from declaring or paying any dividends during any financial year if it is not in compliance with its minimum solvency margin. In addition, Arch Re Bermuda is prohibited from declaring or paying in any financial year dividends of more than 25% of its total statutory capital and surplus (as shown on its previous financial year's statutory balance sheet) unless it files, at least seven days before payment of such dividends, with the Bermuda Monetary Authority an affidavit stating that it will continue to meet the required margins. In addition, Arch Re Bermuda is prohibited, without prior approval of the Bermuda Monetary Authority, from reducing by 15% or more its total statutory capital, as set out in its previous year's financial statements. At December 31, 2003, Arch Re Bermuda had statutory capital and surplus as determined under Bermuda law of $1.43 billion (including interests in U.S. insurance and reinsurance subsidiaries). Accordingly, as of December 31, 2003, 15% of Arch Re Bermuda s capital, or approximately $214.5 million, is available for dividends without prior approval under Bermuda law, as discussed above. Our U.S. insurance and reinsurance subsidiaries, on a consolidated basis, may not pay any significant dividends or distributions during 2004 without prior regulatory approval. In addition, the ability of our insurance and reinsurance subsidiaries to pay dividends could be constrained by our dependence on financial strength ratings from independent rating agencies. Our ratings from these agencies depend to a large extent on the capitalization levels of our insurance and reinsurance subsidiaries.

If our Bermuda reinsurance subsidiary is unable to provide collateral to ceding companies, its ability to conduct business could be significantly and negatively affected.

Arch Re Bermuda is a registered Bermuda insurance company and is not licensed or admitted as an insurer in any jurisdiction in the United States. Because insurance regulations in the United States do not permit insurance companies to take credit for reinsurance obtained from unlicensed or non-admitted insurers on their statutory financial statements unless security is posted, Arch Re Bermuda's contracts generally require it to post a letter of credit or provide other security after a reinsured reports a claim. Although, to date, Arch Re Bermuda has not experienced any difficulties in providing collateral when required, if we are unable to post security in the form of letters of credit or trust funds when required, the operations of Arch Re Bermuda could be significantly and negatively affected.

We may become subject to taxes in Bermuda after March 28, 2016, which may have a material adverse effect on our results of operations.

Foreign currency exchange rate fluctuation may adversely affect our financial results.

We write business on a worldwide basis, and our results of operations may be affected by fluctuations in the value of currencies other than the U.S. dollar. The primary foreign currencies in which we operate are the Euro, the British Pound Sterling and the Canadian Dollar. Changes in foreign currency exchange rates can reduce our revenues and increase our liabilities and costs, as measured in the U.S. dollar as our functional currency. To date, we have not attempted to reduce our exposure to these exchange rate risks by using hedging transactions or by investing in securities denominated in currencies other than the U.S. dollar. We may therefore suffer losses solely as a result of exchange rate fluctuations. In order to minimize the impact of exchange rate fluctuations, we are considering reducing our exposure to these exchange rate risks in 2004 by investing in securities denominated in currencies other than the U.S. dollar. Since inception through December 31, 2003, we have recorded net premiums written of approximately $313.6 million from British Pound Sterling-denominated contracts, $95.8 million from Euro-denominated contracts and $68.9 million from Canadian Dollar-denominated contracts. For the years ended December 31, 2003 and 2002, net foreign exchange gains were $997,000 and $2.4 million, respectively.

Employees of our Bermuda operations are required to obtain work permits before engaging in a gainful occupation in Bermuda. Required work permits may not be granted or may not remain in effect.

Under Bermuda law, non-Bermudians (other than spouses of Bermudians) may not engage in any gainful occupation in Bermuda without an appropriate governmental work permit. Our success may depend in part on the continued services of key employees in Bermuda. A work permit may be granted or renewed upon showing that, after proper public advertisement, no Bermudian (or spouse of a Bermudian or a holder of a permanent resident's certificate or holder of a working resident's certificate) is available who meets the minimum standards reasonably required by the employer. The Bermuda government's policy places a six-year term limit on individuals with work permits, subject to certain exemptions for key employees. A work permit is issued with an expiry date (up to five years) and no assurances can be given that any work permit will be issued or, if issued, renewed upon the expiration of the relevant term. We consider our key officers in Bermuda to be Constantine Iordanou, our President and Chief Executive Officer (work permit expires November 12, 2006), Dwight R. Evans, Chairman and Chief Executive Officer of Arch Re Bermuda (work permit expires May 12, 2005), and John D. Vollaro, our Executive Vice President and Chief Financial Officer (work permit expires July 25, 2005). If work permits are not obtained or renewed for our principal employees, we could lose their services, which could materially affect our business.

The enforcement of civil liabilities against us may be difficult.

obtained in a court in the United States under which a sum of money is payable as compensatory damages (i.e., not being a sum claimed by a revenue authority for taxes or other charges of a similar nature by a governmental authority, or in respect of a fine or penalty or multiple or punitive damages) may be the subject of an action on a debt in the Supreme Court of Bermuda under the common law doctrine of obligation. Such an action should be successful upon proof that the sum of money is due and payable, and without having to prove the facts supporting the underlying judgment, as long as:

•: the court which gave the judgment had proper jurisdiction over the parties to such judgment;
•: such court did not contravene the rules of natural justice of Bermuda;
•: such judgment was not obtained by fraud;
•: the enforcement of the judgment would not be contrary to the public policy of Bermuda;
•: no new admissible evidence relevant to the action is submitted prior to the rendering of the judgment by the courts of Bermuda; and
•: there is due compliance with the correct procedures under Bermuda law.

A Bermuda court may impose civil liability on us or our directors or officers in a suit brought in the Supreme Court of Bermuda against us or such persons with respect to a violation of U.S. federal securities laws, provided that the facts surrounding such violation would constitute or give rise to a cause of action under Bermuda law.

Risks Relating to Taxation

We and our non-U.S. subsidiaries may become subject to U.S. federal income taxation.

ACGL and its non-U.S. subsidiaries intend to operate their business in a manner that will not cause them to be treated as engaged in a trade or business in the United States and, thus, will not be required to pay U.S. federal income taxes (other than withholding taxes on certain U.S. source investment income) on their income. However, because there is uncertainty as to the activities which constitute being engaged in a trade or business in the United States, there can be no assurances that the U.S. Internal Revenue Service will not contend successfully that ACGL or its non-U.S. subsidiaries are engaged in a trade or business in the United States. If ACGL or any of its non-U.S. subsidiaries were subject to U.S. income tax, our shareholders' equity and earnings could be adversely affected. Certain of our U.S. subsidiaries were personal holding companies in respect of 2002 and 2003, but did not have "undistributed personal holding company income".

We changed our legal domicile from the United States to Bermuda in November 2000. Legislation has recently been introduced which (if enacted) could eliminate the tax benefits available to companies, like us, that changed their legal domiciles to Bermuda. In addition, some U.S. insurance companies have been lobbying Congress to pass legislation intended to eliminate certain perceived tax advantages of U.S. insurance companies with Bermuda affiliates resulting principally from reinsurance between or among U.S. insurance companies and their Bermuda affiliates. Legislation, if passed, and other changes in U.S. tax laws, regulations and interpretations thereof to address these issues could adversely affect us.

U.S. persons who hold our common shares may be subject to U.S. income taxation at ordinary income rates on our undistributed earnings and profits. In addition, the heirs or estate of any individual holder may not be entitled to a "step-up" in basis of our common shares which might otherwise be available upon such holder's death.

We believe that we and our non-U.S. subsidiaries currently are controlled foreign corporations ("CFCs"), although our bye-laws are designed to preclude any U.S. person from adverse tax

consequences as a result of our CFC status. ACGL and certain of its non-U.S. subsidiaries were also foreign personal holding companies in respect of 2002 and 2003, but did not have undistributed foreign personal holding company income. We do not believe that we are a passive foreign investment company. Since these determinations and beliefs are based upon legal and factual conclusions, no assurances can be given that the U.S. Internal Revenue Service or a court would concur with our conclusions. If they were not to so concur, U.S. persons who hold our common shares may suffer adverse tax consequences.

RATIO OF EARNINGS TO FIXED CHARGES AND PREFERENCE SHARE DIVIDENDS

The ratio of earnings to fixed charges for each of the periods set forth below is as follows:

		Year Ended December 31,										Three Months Ended March 31,
		1999		2000		2001		2002		2003		2004
Ratio of earnings to fixed charges(a)(b)		N/A(c	)	N/A(c	)	488.5x		50.1x		71.4x		34.7x

(a): For purposes of determining the ratio of earnings to fixed charges, "earnings" consists of (a) income (loss) before income taxes, extraordinary item and cumulative effect of accounting change, minus (b) equity in net income (loss) of investees, plus (c) fixed charges, and "fixed charges" consists of (a) interest and amortization on indebtedness, plus (b) estimate of interest component within rental expense net of sublease income.
(b): Arch Capital has had no dividend bearing preference shares during the period covered by the ratio of earnings to fixed charges and preference share dividends table listed above. Until September 29, 2003, Arch Capital's fixed charges only included the interest component of rental expense net of sublease income.
(c): For the years ended December 31, 2000 and 1999, Arch Capital's income was insufficient by $1,379,000 and $52,309,000, respectively, to cover its fixed charges.

Because Arch US has, and for the previous five fiscal years has had, no registered debt and no dividend bearing preference shares, the ratio of earnings to fixed charges and the ratio of earnings to combined fixed charges and preference share dividends are not applicable.

USE OF PROCEEDS

Except as may otherwise be described in the prospectus supplement relating to an offering of securities, the net proceeds from the sale of the securities included in this prospectus will be used for general corporate purposes. Any specific allocation of the net proceeds of an offering of securities to a specific purpose will be determined at the time of such offering and will be described in the related prospectus supplement.

GENERAL DESCRIPTION OF THE OFFERED SECURITIES

Arch Capital may offer from time to time under this prospectus, separately or together:

•: common shares,
•: preference shares,
•: unsecured senior or subordinated debt securities,
•: depositary shares, each consisting of one or more or a specified fraction of a common share(s) or a preference share(s),
•: warrants to purchase common shares, preference shares or debt securities,
•: share purchase contracts to purchase common shares, and
•: share purchase units, each consisting of (a) an Arch Capital Group Ltd. common share purchase contract, under which the holder or Arch Capital Group Ltd., upon settlement, will purchase a fixed or varying number of Arch Capital Group Ltd. common shares, and (b) a beneficial interest in either Arch Capital Group Ltd. debt securities (which may be senior or subordinated), Arch Capital Group (U.S.) Inc. senior debt securities, Arch Capital Group Ltd. preference shares or debt or equity obligations of third parties, including U.S. Treasury securities, purchased with the proceeds from the sale of the Arch Capital Group Ltd. share purchase units.

Arch Capital Group (U.S.) Inc. may offer from time to time under this prospectus unsecured senior debt securities, which will be fully and unconditionally guaranteed by Arch Capital.

Material U.S. federal income tax considerations pertaining to an investment in the securities offered will be described in the applicable prospectus supplement.

References to "Arch Capital," "we," "our" or "us" in "Description of Arch Capital Common Shares," "Description of Arch Capital Preference Shares," "Description of Arch Capital Debt Securities," "Description of Arch Capital Depositary Shares," "Description of Arch Capital Warrants to Purchase Common Shares or Preference Shares," "Description of Arch Capital Warrants to Purchase Debt Securities" and "Description of Arch Capital Share Purchase Contracts and the Share Purchase Units" refer solely to Arch Capital Group Ltd. and not its subsidiaries.

DESCRIPTION OF ARCH CAPITAL SHARE CAPITAL

The authorized share capital of Arch Capital consists of 200,000,000 common shares, par value U.S. $0.01 per share, and 50,000,000 preference shares, par value U.S. $0.01 per share. Without giving effect to the conversion of all outstanding series A convertible preference shares, as of March 31, 2004, there were 33,552,344 common shares outstanding and 38,364,972 preference shares outstanding. All issued and outstanding shares are fully paid and non-assessable.

DESCRIPTION OF ARCH CAPITAL COMMON SHARES

General

The following description of our common shares is a summary. This summary is not complete and is subject to the complete text of applicable laws and our memorandum of association and bye-laws.

Holders of the common shares have no preemptive, redemption, conversion or sinking fund rights. Subject to the voting restrictions described below, holders of common shares are entitled to one vote per share on all matters submitted to a vote of holders of common shares and do not have any cumulative voting rights. In the event of a liquidation, dissolution, or winding up of the company, the holders of common shares are entitled to share equally and ratably in the assets of the company, if any, remaining after the payment of all debts and liabilities of the company and the liquidation preference of any outstanding preference shares. All outstanding common shares are fully paid and non-assessable. The board is permitted to authorize the issuance of additional common shares. Issuances of common shares are subject to the applicable rules of the Nasdaq National Market or other organizations on whose systems our common shares may then be quoted or listed.

Transfer Agent

American Stock Transfer & Trust Company is the transfer agent and registrar of our common shares.

Dividends

Holders of our common shares are entitled to participate equally in dividends when the board declares dividends on common shares out of funds legally available for dividends. The rights of holders of our common shares to receive dividends are subject to the preferences of holders of our preference shares, including our series A convertible preference shares, which require that no dividends may be paid on or with respect to our common shares prior to the declaration and payment of a dividend with respect to the series A convertible preference shares.

Liquidation Rights

In the event of a liquidation, dissolution, or winding up of the company, the holders of common shares have a right to a ratable portion of assets remaining after the payment of all debts and liabilities of the company, if any, and the liquidation preference of any outstanding preference shares, including, with respect to our outstanding series A convertible preference shares, an amount equal to the liquidation preference of $21.00 per share held by such holder, plus all accumulated and unpaid dividends, before any distribution is made on any common shares.

Voting Limitation

Under our bye-laws, if the votes conferred by our shares directly or indirectly or constructively owned (within the meaning of section 958 of the Internal Revenue Code of 1986, as amended (the "Code")) by any U.S. person (as defined in section 7701(a)(30) of the Code) would otherwise represent more than 9.9% of the voting power of all shares entitled to vote generally at an election of directors, the votes conferred by such shares or such U.S. person will be reduced by whatever amount is necessary so that after any such reduction the votes conferred by the shares of such person will constitute 9.9% of the total voting power of all shares entitled to vote generally at an election of directors.

There may be circumstances in which the votes conferred on a U.S. person are reduced to less than 9.9% as a result of the operation of bye-law 45 because of shares, including shares held by private

equity investment funds affiliated with Warburg Pincus LLC ("Warburg Pincus funds") and Hellman & Friedman LLC ("Hellman & Friedman funds"), that may be attributed to that person under the Code.

Notwithstanding the provisions of our bye-laws described above, after having applied such provisions as best as they consider reasonably practicable, the board may make such final adjustments to the aggregate number of votes conferred by the shares of any U.S. person that they consider fair and reasonable in all the circumstances to ensure that such votes represent 9.9% of the aggregate voting power of the votes conferred by all our shares entitled to vote generally at an election of directors.

In order to implement bye-law 45, we will assume that all shareholders (other than the Warburg Pincus funds and the Hellman & Friedman funds) are U.S. persons unless we receive assurances satisfactory to us that they are not U.S. persons.

Non-U.S. Currency

If the purchase price of any common shares is payable in a currency other than U.S. dollars, the specific terms with respect to such common shares and such foreign currency will be specified in the applicable prospectus supplement.

DESCRIPTION OF ARCH CAPITAL PREFERENCE SHARES

General

The following summary of terms of our preference shares is not complete. You should refer to the provisions of our memorandum of association and bye-laws and the terms of each class or series of the preference shares which will be filed with the SEC at or prior to the time of issuance of such class or series of preference shares and described in the applicable prospectus supplement. The applicable prospectus supplement may also state that any of the terms set forth herein are inapplicable to such series of preference shares, provided that the information set forth in such prospectus supplement does not constitute material changes to the information herein such that it alters the nature of the offering or the securities offered.

Our bye-laws allow the board to authorize the issuance of preference shares in one or more series, and may fix the rights and preferences of those shares, including as to dividends, voting (which will be subject to the limitations described below under "Description of Arch Capital Common Shares—Voting Limitation"), redemption, conversion rights and otherwise.

Issuances of preference shares are subject to the applicable rules of the Nasdaq National Market or other organizations on whose systems our preference shares may then be quoted or listed. Depending upon the terms of preference shares established by our board of directors, any or all series of preference shares could have preferences over the common shares with respect to dividends and other distributions and upon liquidation of the company. Issuance of any such shares with voting powers, or issuance of additional shares of common shares, would dilute the voting power of the outstanding common shares.

Terms

The terms of each series of preference shares will be described in any prospectus supplement related to such series of preference shares.

The board of directors in approving the issuance of a series of preference shares has authority to determine, and the applicable prospectus supplement may set forth with respect to such series, the following terms, among others:

•: the number of shares constituting that series and the distinctive designation of that series;

•: the dividend rate on the shares of that series, if any, whether dividends will be cumulative and, if so, from which date or dates, and the relative rights of priority, if any, of payment of dividends on shares of that series;
•: the voting rights for shares of the series, if any, in addition to the voting rights provided by law, and the terms of such voting rights;
•: the conversion or exchange privileges for shares of the series, if any (including, without limitation, conversion into common shares), and the terms and conditions of such conversion or exchange, including provisions for adjustment of the conversion or exchange rate in such events as the board will determine;
•: whether or not the shares of that series will be redeemable and, if so, the terms and conditions of such redemption, including the manner of selecting shares for redemption if less than all shares are to be redeemed, the date or dates upon or after which they will be redeemable, and the amount per share payable in case of redemption, which amount may vary under different conditions and at different redemption dates;
•: any sinking fund for the redemption or purchase of shares of that series and the terms and amount of such sinking fund;
•: the right of the shares of that series to the benefit of conditions and restrictions upon the creation of indebtedness of Arch Capital or any of our subsidiaries, upon the issue of any additional shares (including additional shares of such series or any other series) and upon the payment of dividends or the making of other distributions on, and the purchase, redemption or other acquisition by us or any of our subsidiaries of, any of our outstanding shares;
•: the rights of the shares of that series in the event of our voluntary or involuntary liquidation, dissolution or winding up, and the relative rights of priority, if any, of payment of shares of that series; and
•: any other relative participating, optional or other special rights, qualifications, limitations or restrictions of that series.

Series A Convertible Preference Shares

Set forth below is a summary of the material terms of the certificate of designations for our series A convertible preference shares.

Dividends

The holders of our preference shares are entitled to receive dividends along with holders of our common shares on an as converted basis. Dividends are payable on each outstanding preference share on an as-converted basis. No dividends may be paid or declared on or with respect to our common shares prior to the declaration and payment of a dividend on or with respect to the preference shares. Dividends on the preference shares are noncumulative.

Ranking

The preference shares rank, with respect to dividends and upon liquidation, winding up or dissolution on a parity with those shares which, by the terms of our bye-laws or by such rights, preferences or limitations as fixed by the board of directors, are specifically entitled to share ratably with the preference shares, and senior to each other class or series of our shares.

The shareholders agreement between us and the holders of our preference shares include a limitation on dividends on, and repurchases of, our common shares.

Conversion

General. Each preference share is convertible at any time and from time to time at the option of the holder, initially, into one fully paid and nonassessable common share, subject to adjustment for certain events. In connection with the conversion of any preference shares, no fractional shares will be issued. Instead, we will pay a cash amount for such fractional interest based on the current market price of the common shares.

Mandatory Conversion. The preference shares will automatically convert into common shares 90 days following the consummation of the purchase price adjustment to be performed on November 20, 2005.

Adjustments. The number of common shares into which each preference share is convertible is subject to adjustment from time to time in the event of:

•: share splits or combinations of our common shares;
•: the declaration and payment of dividends on the common shares in additional common shares;
•: the distribution of indebtedness, securities or assets to holders of our common shares;
•: transactions in which our common shares are exchanged (either for different securities of our company or securities of a different company); or
•: offerings of our common shares, or securities convertible into our common shares, at a price below the market value for our common shares at the time of issuance, subject to exceptions.

Liquidation Preference. In the event of our voluntary or involuntary liquidation, dissolution or winding-up, or a reduction or decrease in our share capital resulting in a distribution of assets to the holders of any class or series of our shares, each holder of preference shares is entitled to payment out of our assets available for distribution of an amount equal to the liquidation preference of $21.00 per preference share held by such holder, plus all accumulated and unpaid dividends, before any distribution is made on any common shares. If, in the event of our voluntary or involuntary liquidation, dissolution or winding-up or a reduction or decrease in our share capital, the amounts payable with respect to preference shares and parity shares are not paid in full, the holders of preference shares and the parity shares share equally and ratably in any distribution of shares assets in proportion to the full liquidation preference and all accumulated and unpaid dividends to which each such holder is entitled.

Voting Rights. Each holder of preference shares is entitled to the number of votes equal to the number of whole common shares into which all of such holder's preference shares are convertible, with respect to all matters submitted for shareholder approval. Except as required by applicable Bermuda law or by the express terms of the preference shares, holders will vote together with holders of the common shares as a single class.

The affirmative vote of the holders of at least a majority of the outstanding preference shares, voting with holders of shares of all other series of preference shares affected in the same way as a single class, is required to amend, repeal or change any provisions of the certificate of designations in any manner which would adversely affect, alter or change the powers, preferences or special rights of the preference shares and any such securities affected in the same way. However, the creation, authorization or issuance of any other class or series of shares or the increase or decrease in the amount of authorized shares of any class or series or of the preference shares, or any increase, decrease or change in the par value of any class or series of shares (including the preference shares) do not require the consent of the holders of the preference shares and are not deemed to affect adversely, alter or change the powers, preferences and special rights of the preference shares. With respect to any matter on which the holders are entitled to vote as a separate class, each preference share is entitled to one vote.

Non-U.S. Currency

If the purchase price of any preference shares is payable in a currency other than U.S. dollars, the specific terms with respect to such preference shares and such foreign currency will be specified in the applicable prospectus supplement.

DESCRIPTION OF ARCH CAPITAL DEBT SECURITIES

General

We may issue debt securities from time to time in one or more series, under one or more indentures, each dated as of a date on or prior to the issuance of the debt securities to which it relates. Senior debt securities and subordinated debt securities may be issued pursuant to separate indentures, a senior indenture and a subordinated indenture, respectively, in each case between us and a trustee qualified under the Trust Indenture Act. The form of such indentures have been filed as an exhibit to the registration statement of which this prospectus is a part, subject to such amendments or supplements as may be adopted from time to time. The senior indenture and the subordinated indenture, as amended or supplemented from time to time, are sometimes referred to individually as an "indenture" and collectively as the "indentures." Each indenture will be subject to and governed by the Trust Indenture Act. The aggregate principal amount of debt securities which may be issued under each indenture will be unlimited and each indenture will set forth the specific terms of any series of debt securities or provide that such terms will be set forth in, or determined pursuant to, an authorizing resolution, as defined in the applicable prospectus supplement, and/or a supplemental indenture, if any, relating to such series.

The statements made below relating to the debt securities and the indentures are summaries of the anticipated provisions thereof, do not purport to be complete and are subject to, and are qualified in their entirety by reference to, all of the provisions of the applicable indenture and any applicable U.S. federal income tax considerations as well as any applicable modifications of or additions to the general terms described below in the applicable prospectus supplement. The applicable prospectus supplement may also state that any of the terms set forth herein are inapplicable to such series of debt securities; provided that the information set forth in such prospectus supplement does not constitute material changes to the information herein such that it alters the nature of the offering or the securities offered.

Terms

The debt securities will be our unsecured obligations.

The senior debt securities will rank equal in right of payment with all our other unsecured and unsubordi-nated indebtedness.

The subordinated debt securities will be subordinated in right of payment to the prior payment in full of all our senior indebtedness, which is defined in the section called "—Ranking of Debt Securities" below.

The specific terms of each series of debt securities will be set forth in the applicable prospectus supplement relating thereto, including the following, as applicable:

(1) the title of such debt securities and whether such debt securities are senior debt securities or subordinated debt securities and, if subordinated debt securities, the specific subordination provisions applicable thereto;

(2) the aggregate principal amount of such debt securities and any limit on such aggregate principal amount;

(3) if the price (expressed as a percentage of the principal amount thereof) at which such debt securities will be issued is an amount other than the principal amount thereof, the portion of the principal amount thereof payable upon declaration of acceleration of the maturity thereof, or if applicable, the portion of the principal amount of such debt securities that is convertible into common shares or preference shares or the method by which any such portion will be determined;

(4) if convertible into common shares or preference shares, the terms on which such debt securities are convertible, including the initial conversion price, the conversion period, any events requiring an adjustment of the applicable conversion price and any requirements relating to the reservation of such common shares or preference shares for purposes of conversion;

(5) the date(s), or the method for determining such date or dates, on which the principal of such debt securities will be payable and, if applicable, the terms on which such maturity may be extended;

(6) the rate(s) (which may be fixed or floating), or the method by which such rate or rates will be determined, at which such debt securities will bear interest, if any, including if applicable, that such debt securities will bear interest at an increased rate (up to a specified maximum) upon the occurrence of an event of default and/or under certain circumstances described in the applicable prospectus supplement (which may include, among other things, a reduction in the trading price of our common shares below certain levels for a minimum period of time);

(7) the date(s), or the method for determining such date or dates, from which any such interest will accrue, the dates on which any such interest will be payable, the record dates for such interest payment dates, or the method by which such dates will be determined, the persons to whom such interest will be payable, and the basis upon which interest will be calculated if other than that of a 360-day year of twelve 30-day months;

(8) the place(s) where the principal of and interest, if any, on such debt securities will be payable, where such debt securities may be surrendered for registration of transfer or exchange and where notices or demands to or upon us in respect of such debt securities and the applicable indenture may be served;

(9) the period(s), if any, within which, the price or prices at which and the other terms and conditions upon which such debt securities may, pursuant to any optional or mandatory redemption provisions, be redeemed, as a whole or in part, at our option;

(10) our obligation, if any, to redeem, repay or repurchase such debt securities pursuant to any sinking fund (as defined in the applicable supplemental indenture) or analogous provision or at the option of a holder thereof, and the period or periods within which, the price or prices at which and the other terms and conditions upon which such debt securities will be redeemed, repaid or purchased, as a whole or in part, pursuant to such obligations;

(11) if other than U.S. dollars, the currency or currencies in which the principal of and interest, if any, on such debt securities are denominated and payable, which may be a foreign currency or units of two or more foreign currencies or a composite currency or currencies, and the terms and conditions relating thereto;

(12) whether the amount of payments of principal of or interest, if any, on such debt securities may be determined with reference to an index, formula or other method (which index, formula or method may, but need not be, based on the yield on or trading price of other securities, including United States Treasury securities, or on a currency, currencies, currency unit or units, or composite currency or currencies) and the manner in which such amounts will be determined;

(13) whether the principal of or interest, if any, on the debt securities of the series is to be payable, at our election or a holder thereof, in a currency or currencies, currency unit or units or

composite currency or currencies other than that in which such debt securities are denominated or stated to be payable and the period or periods within which, and the terms and conditions upon which, such election may be made;

(14) provisions, if any, granting special rights to the holders of debt securities of the series upon the occurrence of such events as may be specified;

(15) any deletions from, modifications of or additions to the events of default or our covenants with respect to debt securities of the series, whether or not such events of default or covenants are consistent with the events of default or covenants described herein;

(16) whether debt securities of the series are to be issuable initially in temporary global form and whether any debt securities of the series are to be issuable in permanent global form and, if so, whether beneficial owners of interests in any such security in permanent global form may exchange such interests for debt securities of such series and of like tenor of any authorized form and denomination and the circumstances under which any such exchanges may occur, if other than in the manner provided in the applicable indenture, and, if debt securities of the series are to be issuable as a global security, the identity of the depository for such series;

(17) the applicability, if any, of the defeasance and covenant defeasance provisions of the applicable indenture to the debt securities of the series;

(18) if exchangeable into another series of debt securities of Arch Capital, the terms on which such debt securities are exchangeable; and

(19) any other terms of the series of debt securities and any additions, deletions or modifications to the applicable indenture.

The debt securities, if convertible or exchangeable, will not be convertible into or exchangeable for securities of a third party.

If the applicable prospectus supplement provides, the debt securities may be issued at a discount below their principal amount and provide for less than the entire principal amount thereof to be payable upon declaration of acceleration of the maturity thereof.

Except as may be set forth in the applicable prospectus supplement, the debt securities will not contain any provisions that would limit our ability to incur indebtedness or that would afford holders of debt securities protections against transactions involving us, including a highly leveraged transaction involving us or a change of control. The applicable prospectus supplement will contain information with respect to any deletions from, modifications of or additions to the events of default or covenants described below, including any addition of a covenant or other provision providing event risk or similar protection.

Denomination, Interest, Registration and Transfer

We will issue the debt securities of each series only in registered form, without coupons, in denominations of $1,000, or in such other currencies or denominations as may be set forth in the applicable supplemental indenture or specified in, or pursuant to, an authorizing resolution and/or supplemental indenture, if any, relating to such series of debt securities.

The principal of and interest, if any, on any series of debt securities will be payable at the corporate trust office of the trustee, the address of which will be stated in the applicable prospectus supplement. However, at our option, interest payments may be made by check mailed to the address of the person entitled thereto as it appears in the applicable register for such debt securities.

Subject to certain limitations imposed upon debt securities issued in book-entry form, the debt securities of any series:

•: will be exchangeable for any authorized denomination of other debt securities of the same series and of a like aggregate principal amount and tenor upon surrender of such debt securities at the trustee's corporate trust office or at the office of any registrar designated by us for such purpose; and
•: may be surrendered for registration of transfer or exchange thereof at the corporate trust office of the trustee or at the office of any registrar designated by us for such purpose.

No service charge will be made for any registration of transfer or exchange, but we may require payment of a sum sufficient to cover any tax or other governmental charge payable in connection with certain transfers and exchanges. We may act as registrar and may change any registrar without notice.

Certain Covenants

The applicable prospectus supplement will describe any material covenants in respect of a series of debt securities that are not described in this prospectus.

Unless otherwise indicated in the applicable prospectus supplement, senior debt securities and the subordinated debt securities will include the provision described below.

Merger, Amalgamation, Consolidation or Sale of Assets

We may not consolidate or amalgamate with or merge with or into any other person (other than a subsidiary) or convey, transfer, sell or lease our properties and assets substantially as an entirety to any person (other than a subsidiary), or permit any person (other than a subsidiary) to consolidate with or merge into us, unless:

•: either (a) we shall be the surviving person or (b) the surviving person (if other than us) shall (1) be a corporation or limited liability company organized and existing under the laws of the United States of America, any state thereof, the District of Columbia, Bermuda, the Cayman Islands or any country or state which is a member of the Organization of Economic Cooperation and Development and (2) expressly assume, by an indenture supplemental to the indenture, executed and delivered to the trustee, in form reasonably satisfactory to the trustee, all of our obligations under the debt securities and the indenture;
•: immediately after giving effect to such transaction, no event of default, and no event that, after notice or lapse of time or both, would become an event of default, shall have occurred and be continuing; and
•: we shall have delivered to the trustee an officers' certificate stating that such consolidation, merger, conveyance, transfer, sale or lease and, if a supplemental indenture is required in connection with such transaction, such supplemental indenture, comply with the base indenture and that all conditions precedent herein provided for relating to such transaction have been satisfied.

The surviving person of such transaction shall succeed to, and be substituted for, and may exercise every right and power of, us under the indenture with the same effect as if such successor had been named as us thereunder; and thereafter, we shall be discharged from all obligations and covenants under the base indenture and the debt securities.

Payment of Principal, Premium and Interest

We will duly and punctually pay the principal of (and premium, if any) and interest on the debt securities in accordance with their terms.

Maintenance of Office or Agency

We will maintain an office or agency where the debt securities may be presented or surrendered for registration of transfer or exchange and where notices and demands to or upon us in respect of the debt securities.

Money for Securities; Payments to Be Held in Trust

If we will at any time act as our own paying agent with respect to any debt securities, we will, on or before each due date of the principal of (and premium, if any) or interest on such debt securities, segregate and hold in trust for the benefit of the persons entitled thereto a sum sufficient to pay the principal (and premium, if any) or interest so becoming due until such sums will be paid to such persons or otherwise disposed of as provided in the indentures and will promptly notify the trustee of our action or failure so to act.

Corporate Existence

Except as otherwise permitted under "—Merger, Amalgamation, Consolidation or Sale of Assets" above, we will do or cause to be done all things necessary to preserve and keep in full force and effect our corporate existence and material rights (charter and statutory) and franchises; provided, however, that we will not be required to preserve any such right or franchise if our board of directors or senior management determines that the preservation thereof is no longer desirable in the conduct of our business and that the loss thereof is not disadvantageous in any material respect to the holders of debt securities.

Statement by Officers as to Default

We will deliver to the trustee, within 120 days after the end of each fiscal year of the Company, a certificate of our principal executive officer, principal financial officer or principal accounting officer stating whether or not to the best knowledge of the signer thereof we are in default in the performance and observance of any of the terms, provisions and conditions of the indenture, and if we are in default, specifying all such defaults and the nature and status thereof of which they may have knowledge.

Waiver of Certain Covenants

We may omit in any particular instance to comply with any term, provision or condition of the covenants described above under the captions "—Corporate Existence" and "—Statement by Officers as to Default" if before or after the time for such compliance the holders of at least a majority in principal amount of the outstanding debt securities (taken together as one class) will, by act of such holders, either waive such compliance in such instance or generally waive compliance with such term, provision or condition except to the extent so expressly waived, and, until such waiver will become effective, our obligations and the duties of the trustee in respect of any such term, provision or condition will remain in full force and effect.

Ranking of Debt Securities

General

We currently conduct substantially all of our operations through our subsidiaries and our subsidiaries generate substantially all of our operating income and cash flow. As a result, distributions and advances from our subsidiaries will be the principal source of funds necessary to meet our debt service obligations. Contractual provisions or laws, as well as our subsidiaries financial condition and operating and regulatory requirements, may limit our ability to obtain cash from our subsidiaries that we require to pay our debt service obligations. In addition, because we are a holding company holders of the debt securities will have a junior position to the claims of creditors of our subsidiaries on their assets and earnings.

Senior Debt Securities

The senior debt securities will be our unsecured unsubordinated obligations and will:

•: rank equal in right of payment with all our other unsecured and unsubordinated indebtedness;
•: be effectively subordinated in right of payment to all our secured indebtedness to the extent of the value of the assets securing such indebtedness; and
•: be effectively subordinated to all of our subsidiaries' indebtedness.

As of March 31, 2004 (after giving effect to our offering of $300.0 million aggregate principal amount of our 7.35% Senior Notes due 2034 in May 2004 and the application of the proceeds therefrom), the aggregate amount of our outstanding consolidated indebtedness for money borrowed was approximately $300.0 million, of which none was secured and all would rank equal in right of payment with the senior debt that may be issued under the senior debt securities indenture. All obligations (including insurance obligations of our subsidiaries would be effectively senior to any senior or subordinated debt issued by Arch Capital or Arch US. As of March 31, 2004, the consolidated obligations of our subsidiaries reflected on our balance sheet was approximately $4.3 billion.

Except as otherwise set forth in the applicable senior indenture or specified in an authorizing resolution and/or supplemental indenture, if any, relating to a series of senior debt securities to be issued, there will be no limitations in any senior indenture on the amount of additional indebtedness which may rank equal with the senior debt securities or on the amount of indebtedness, secured or otherwise, which may be incurred by any of our subsidiaries.

Subordinated Debt Securities

The subordinated debt securities will be our unsecured subordinated obligations. Unless otherwise provided in the applicable prospectus supplement, the payment of principal of, interest on and all other amounts owing in respect of the subordinated debt securities will be subordinated in right of payment to the prior payment in full in cash of principal of, interest on and all other amounts owing in respect of all of our senior indebtedness. Upon any payment or distribution of our assets of any kind or character, whether in cash, property or securities, to creditors upon any total or partial liquidation, dissolution, winding-up, reorganization, assignment for the benefit of creditors or marshaling of our assets or in a bankruptcy, reorganization, insolvency, receivership or other similar proceeding relating to us or our property, whether voluntary or involuntary, all principal of, interest on and all other amounts due or to become due will be paid, first, to all senior indebtedness in full in cash, or such payment duly provided for to the satisfaction of the holders of senior indebtedness, before any payment or distribution of any kind or character is made on account of any principal of, interest on or other amounts owing in respect of the subordinated debt securities, or for the acquisition of any of the subordinated debt securities for cash, property or otherwise.

If any default occurs and is continuing in the payment when due, whether at maturity, upon any redemption, by declaration or otherwise, of any principal of, interest on, unpaid drawings for letters of credit issued in respect of, or regularly accruing fees with respect to, any senior indebtedness, no payment of any kind or character will be made by or on behalf of us or any other person on our or their behalf with respect to any principal of, interest on or other amounts owing in respect of the subordinated debt securities or to acquire any of the subordinated debt securities for cash, property or otherwise.

As of March 31, 2004 (after giving effect to our offering of $300.0 million aggregate principal amount of our 7.35% Senior Notes due 2034 in May 2004 and the application of the proceeds therefrom), the aggregate amount of our consolidated indebtedness for money borrowed was approximately $300.0 million, of which all would rank senior in right of payment to the subordinated debt issuable under the subordinated debt securities indenture. All obligations (including insurance obligations of our subsidiaries would be effectively senior to any senior or subordinated debt issued by Arch Capital or Arch US. As of March 31, 2004, the consolidated obligations of our subsidiaries reflected on our balance sheet was approximately $4.3 billion.

If any other event of default occurs and is continuing with respect to any designated senior indebtedness, as such event of default is defined in the instrument creating or evidencing such designated senior indebtedness, permitting the holders of such designated senior indebtedness then outstanding to accelerate the maturity thereof and if the representative (as defined in the applicable indenture) for the respective issue of designated senior indebtedness gives written notice of the event of default to the trustee (a "default notice"), then, unless and until all events of default have been cured or waived or have ceased to exist or the trustee receives notice from the representative for the respective issue of designated senior indebtedness terminating the blockage period (as defined below), during the 179 days after the delivery of such default notice (the "blockage period"), neither we nor any other person on our behalf will:

(1) make any payment of any kind or character with respect to any principal of, interest on or other amounts owing in respect of the subordinated debt securities; or

(2) acquire any of the subordinated debt securities for cash, property or otherwise.

Notwithstanding anything herein to the contrary, in no event will a blockage period extend beyond 179 days from the date the payment on the subordinated debt securities was due and only one such blockage period may be commenced within any 360 consecutive days. No event of default which existed or was continuing on the date of the commencement of any blockage period with respect to the designated senior indebtedness will be, or be made, the basis for commencement of a second blockage period by the representative of such designated senior indebtedness whether or not within a period of 360 consecutive days unless such event of default will have been cured or waived for a period of not less than 90 consecutive days (it being acknowledged that any subsequent action, or any breach of any financial covenants for a period commencing after the date of commencement of such blockage period that, in either case, would give rise to an event of default pursuant to any provisions under which an event of default previously existed or was continuing will constitute a new event of default for this purpose).

As a result of the foregoing provisions, in the event of our insolvency, holders of the subordinated debt securities may recover ratably less than our general creditors.

"Senior indebtedness," unless otherwise specified in one or more applicable supplemental indentures or approved pursuant to a board resolution in accordance with the applicable indenture, means, with respect to us:

(1) the principal (including redemption payments), premium, if any, interest and other payment obligations in respect of (a) our indebtedness for money borrowed and (b) our

indebtedness evidenced by securities, debentures, bonds, notes or other similar instruments issued by us, including any such securities issued under any deed, indenture or other instrument to which we are a party (including, for the avoidance of doubt, indentures pursuant to which senior debt securities have been or may be issued);

(2) all of our obligations issued or assumed as the deferred purchase price of property, all of our conditional sale obligations, all of our hedging agreements and agreements of a similar nature thereto and all agreements relating to any such agreements, and all of our obligations under any title retention agreement (but excluding trade accounts payable arising in the ordinary course of business);

(3) all of our obligations for reimbursement on any letter of credit, banker's acceptance, security purchase facility or similar credit transaction;

(4) all obligations of the type referred to in clauses (1) through (3) above of other persons for the payment of which we are responsible or liable as obligor, guarantor or otherwise;

(5) all obligations of the type referred to in clauses (1) through (4) above of other persons secured by any lien on any of our property or asset (whether or not such obligation is assumed by us); and

(6) any deferrals, amendments, renewals, extensions, modifications and refundings of all obligations of the type referred to in clauses (1) through (5) above, in each case whether or not contingent and whether outstanding at the date of effectiveness of the applicable indenture or thereafter incurred,

except, in each case, for the subordinated debt securities and any such other indebtedness or deferral, amendment, renewal, extension, modification or refunding that contains express terms, or is issued under a deed, indenture or other instrument, which contains express terms, providing that it is subordinate to or ranks equal with the subordinated debt securities.

Such senior indebtedness will continue to be senior indebtedness and be entitled to the benefits of the subordination provisions of the applicable indenture irrespective of any amendment, modification or waiver of any term of such senior indebtedness and notwithstanding that no express written subordination agreement may have been entered into between the holders of such senior indebtedness and the trustee or any of the holders.

Discharge and Defeasance

Under the terms of the indenture, we will be discharged from any and all obligations in respect of the debt securities of any series and the applicable indenture (except in each case for certain obligations to register the transfer or exchange of debt securities, replace stolen, lost or mutilated debt securities, maintain paying agencies and hold moneys for payment in trust) if we deposit with the applicable trustee, in trust, moneys or U.S. government obligations in an amount sufficient to pay all the principal of, and interest on, the debt securities of such series on the dates such payments are due in accordance with the terms of such debt securities.

In addition, unless the applicable prospectus supplement and supplemental indenture provide otherwise, we may elect either (1) to defease and be discharged from any and all obligations with respect to such debt securities ("defeasance") or (2) to be released from our obligations with respect to such debt securities under certain covenants in the applicable indenture, and any omission to comply

with such obligations will not constitute a default or an event of default with respect to such debt securities ("covenant defeasance"):

(1) by delivering all outstanding debt securities of such series to the trustee for cancellation and paying all sums payable by it under such debt securities and the indenture with respect to such series; or

(2) after giving notice to the trustee of our intention to defease all of the debt securities of such series, by irrevocably depositing with the trustee or a paying agent

(a) in the case of any debt securities of any series denominated in U.S. dollars, cash or U.S. government obligations sufficient to pay all principal of and interest on such debt securities; and

(b) in the case of any debt securities of any series denominated in any currency other than U.S. dollars, an amount of the applicable currency in which the debt securities are denominated sufficient to pay all principal of and interest on such debt securities.

Such a trust may only be established if, among other things:

(1) the applicable defeasance or covenant defeasance does not result in a breach or violation of, or constitute a default under or any material agreement or instrument to which we are a party or by which we are bound;

(2) no event of default or event which with notice or lapse of time or both would become an event of default with respect to the debt securities to be defeased will have occurred and be continuing on the date of establishment of such a trust after giving effect to such establishment; and

(3) we have delivered to the trustee an opinion of counsel (as specified in the applicable supplemental indenture) to the effect that the holders will not recognize income, gain or loss for United States federal income tax purposes as a result of such defeasance or covenant defeasance and will be subject to United States federal income tax on the same amounts, in the same manner and at the same times as would have been the case if such defeasance or covenant defeasance had not occurred, and such opinion of counsel, in the case of defeasance, must refer to and be based upon a letter ruling of the Internal Revenue Service received by us, a Revenue Ruling published by the Internal Revenue Service or a change in applicable United States federal income tax law occurring after the date of the applicable supplemental indenture.

In the event we effect covenant defeasance with respect to any debt securities and such debt securities are declared due and payable because of the occurrence of any event of default, other than an event of default with respect to any covenant as to which there has been covenant defeasance, the government obligations on deposit with the trustee will be sufficient to pay amounts due on such debt securities at the time of the stated maturity but may not be sufficient to pay amounts due on such debt securities at the time of the acceleration resulting from such event of default.

Modification and Waiver

We, when authorized by a board resolution, and the trustee may modify, amend and/or supplement the applicable indenture and the applicable debt securities with the consent of the holders of not less than a majority in principal amount of the outstanding debt securities of all series affected thereby (voting as a single class); provided, however, without the consent of each holder of the debt securities

affected thereby, such amendment, modification or supplement may not (with respect to any debt securities held by a non-consenting holder):

(1) change the stated maturity of the principal of or any installment of interest with respect to the debt securities;

(2) reduce the principal amount of, or the rate of interest on, the debt securities;

(3) change the currency of payment of principal of or interest on the debt securities;

(4) impair the right to institute suit for the enforcement of any payment on or with respect to the debt securities;

(5) reduce the above-stated percentage of holders of the debt securities of any series necessary to modify or amend the indenture relating to such series;

(6) modify the foregoing requirements or reduce the percentage of outstanding debt securities necessary to waive any covenant or past default;

(7) in the case of any subordinated indenture, modify the subordination provisions thereof in a manner adverse to the holders of subordinated debt securities of any series then outstanding; or

(8) in the case of any convertible debt securities, adversely affect the right of a holder to convert the debt securities other than as provided in or under the applicable indenture.

Holders of not less than a majority in principal amount of the outstanding debt securities of all series affected thereby (voting as a single class) may waive certain past defaults and may waive compliance by us with any provision of the indenture relating to such debt securities (subject to the immediately preceding sentence); provided, however, that:

(1) without the consent of each holder of debt securities affected thereby, no waiver may be made of a default in the payment of the principal of or interest on any debt security or in respect of a covenant or provision of the indenture that expressly states that it cannot be modified or amended without the consent of each holder affected; and

(2) only the holders of a majority in principal amount of debt securities of a particular series may waive compliance with a provision of the indenture relating to such series or the debt securities of such series having applicability solely to such series.

We, when authorized by a board resolution, and the trustee may amend or supplement the indentures or waive any provision of such indentures and the debt securities without the consent of any holders of debt securities in some circumstance, including:

•: to cure any ambiguity, omission, defect or inconsistency;
•: to make any other change that does not, in the good faith opinion of our board of directors and the trustee, adversely affect the interests of holders of such debt securities in any material respect;
•: to provide for the assumption of our obligations under the applicable indenture by a successor upon any merger, consolidation or asset transfer permitted under the applicable indenture;
•: to provide any security for or guarantees of such debt securities;
•: to add events of default with respect to such debt securities;
•: to add covenants that would benefit the holders of such debt securities or to surrender any rights or powers we have under the applicable indenture;

•: to make any change necessary for the registration of the debt securities under the Securities Act or to comply with the Trust Indenture Act of 1939, or any amendment thereto, or to comply with any requirement of the SEC in connection with the qualification of the applicable indenture under the Trust Indenture Act of 1939; provided, however, that such modification or amendment does not, in the good faith opinion of our board of directors and the trustee, adversely affect the interests of the holders of such debt securities in any material respect;
•: to provide for uncertificated debt securities in addition to or in place of certificated debt securities or to provide for bearer debt securities;
•: to add to or change any of the provisions of the applicable indenture to such extent as will be necessary to permit or facilitate the issuance of the debt securities in bearer form, registrable or not regis-trable as to principal, and with or without interest coupons;
•: to change or eliminate any of the provisions of the applicable indenture; provided, however, that any such change or elimination will become effective only when there is no debt security outstanding of any series created prior to the execution of such supplemental indenture which is entitled to the benefit of such provision;
•: to establish the form or terms of debt securities of any series as permitted by the applicable indenture; or
•: to evidence and provide for the acceptance of appointment by a successor trustee with respect to the debt securities of one or more series and to add to or change any of the provisions of the applicable indenture as will be necessary to provide for or facilitate the administration of the trusts under the applicable indenture by more than one trustee, pursuant to the requirements of the applicable indenture.

Events of Default and Notice Thereof

The following are events that we anticipate will constitute "events of default" with respect to any series of debt securities issued thereunder:

(1) default in the payment of any interest upon any debt securities of that series when it becomes due and payable, and continuance of such default for a period of 30 days; or

(2) default in the payment of the principal of (or premium, if any, on) any debt securities of that series when due; or

(3) default in the deposit of any sinking fund payment, when and as due by the terms of any debt securities of that series; or

(4) default in the performance, or breach, of any covenants of the Company (other than those set forth in clause (1) or (2) above) in the indenture (other than a covenant or warranty added to the indenture solely for the benefit of another series of debt securities) for a period of 60 days after there has been given, and continuance of such by registered or certified mail, to the Company by the trustee or to the Company and the trustee by the holders of at least 25% in principal amount of the outstanding debt securities a written notice specifying such default or breach and requiring it to be remedied and stating that such notice is a Notice of Default hereunder; or

(5) certain events of bankruptcy, insolvency or reorganization.

Additional or different events of default, if any, applicable to the series of debt securities in respect of which this prospectus is being delivered will be specified in the applicable prospectus supplement, and if necessary, a post-effective amendment to this registration statement.

The trustee under such indenture will, within 90 days after the occurrence of any default (the term default to include the events specified above without grace or notice) with respect to any series of debt securities actually known to it, give to the holders of such debt securities notice of such default; provided, however, that, except in the case of a default in the payment of principal of or interest on any of the debt securities of such series or in the payment of a sinking fund installment, the trustee for such series will be protected in withholding such notice if it in good faith determines that the withholding of such notice is in the interest of the holders of such debt securities; and provided, further, that in the case of any default of the character specified in clause (2) above with respect to debt securities of such series, no such notice to holders of such debt securities will be given until at least 30 days after the occurrence thereof. We will certify to the trustee quarterly as to whether any default exists.

If an event of default, other than an event of default resulting from bankruptcy, insolvency or reorganization, with respect to any series of debt securities will occur and be continuing, the trustee for such series or the holders of at least 25% in aggregate principal amount of the debt securities of such series then outstanding, by notice in writing to us (and to the trustee for such series if given by the holders of the debt securities of such series), will be entitled to declare all unpaid principal of and accrued interest on such debt securities then outstanding to be due and payable immediately.

In the case of an event of default resulting from certain events of bankruptcy, insolvency or reorganization, all unpaid principal of and accrued interest on all debt securities of such series then outstanding will be due and payable immediately without any declaration or other act on the part of the trustee for such series or the holders of any debt securities of such series.

Such acceleration may be annulled and past defaults (except, unless theretofore cured, a default in payment of principal of or interest on the debt securities of such series) may be waived by the holders of a majority in principal amount of the debt securities of such series then outstanding upon the conditions provided in the applicable indenture.

No holder of the debt securities of any series issued thereunder may pursue any remedy under such indenture unless the trustee for such series will have failed to act after, among other things, notice of an event of default and request by holders of at least 25% in principal amount of the debt securities of such series of which the event of default has occurred and the offer to the trustee for such series of indemnity satisfactory to it; provided, however, that such provision does not affect the right to sue for enforcement of any overdue payment on such debt securities.

Conversion and Exchange Rights

The terms and conditions, if any, upon which the debt securities of any series will be convertible into common shares or preference shares or upon which the senior debt securities of any series will be exchangeable into another series of debt securities will be set forth in the prospectus supplement relating thereto. Such terms will include the conversion or exchange price (or manner of calculation thereof), the conversion or exchange period, provisions as to whether conversion or exchange will be at the option of the holders of such series of debt securities or at our option or automatic, the events requiring an adjustment of the conversion or exchange price and provisions affecting conversion or exchange in the event of the redemption of such series of debt securities. The debt securities, if convertible or exchangeable, will not be convertible into or exchangeable for securities of a third party.

The Trustee

Subject to the terms of the applicable indenture, the trustee for each series of debt securities will be named in the prospectus supplement relating to each issuance of debt securities. Each indenture will contain certain limitations on a right of the trustee, as our creditor, to obtain payment of claims in certain cases, or to realize on certain property received in respect of any such claim as security or

otherwise. The trustee will be permitted to engage in other transactions; provided, however, that if it acquires any conflicting interest, it must eliminate such conflict or resign.

Subject to the terms of the applicable indenture, the holders of a majority in principal amount of all outstanding debt securities of a series (or if more than one series is affected thereby, of all series so affected, voting as a single class) will have the right to direct the time, method and place of conducting any proceeding for exercising any remedy or power available to the trustee for such series or all such series so affected.

In case an event of default will occur (and will not be cured) under any indenture relating to a series of debt securities and is actually known to a responsible officer of the trustee for such series, such trustee will exercise such of the rights and powers vested in it by such indenture and use the same degree of care and skill in its exercise as a prudent person would exercise or use under the circumstances in the conduct of his own affairs. Subject to such provisions, the trustee will not be under any obligation to exercise any of its rights or powers under the applicable indenture at the request of any of the holders of debt securities unless they will have offered to the trustee security and indemnity satisfactory to it.

Governing Law

The indentures and the debt securities will be governed by the laws of the State of New York.

Global Securities; Book-Entry System

We may issue the debt securities of any series in whole or in part in the form of one or more global securities to be deposited with, or on behalf of, a depository (the "depository") identified in the prospectus supplement relating to such series. "Global securities" represent in the aggregate the total principal or face amount of the securities and once on deposit with a depository, allow trading of the securities through the depository s book-entry system as further described below). Global securities, if any, issued in the United States are expected to be deposited with The Depository Trust Company ("DTC"), as depository. Global securities will be issued in fully registered form and may be issued in either temporary or permanent form. Unless and until it is exchanged in whole or in part for the individual debt securities represented thereby, a global security may not be transferred except as a whole by the depository for such global security to a nominee of such depository or by a nominee of such depository to such depository or another nominee of such depository or by such depository or any nominee of such depository to a successor depository or any nominee of such successor.

The specific terms of the depository arrangement with respect to any series of debt securities will be described in the prospectus supplement relating to such series. We expect that unless otherwise indicated in the applicable prospectus supplement, the following provisions will apply to depository arrangements.

Upon the issuance of a global security, the depository for such global security or its nominee will credit on its book-entry registration and transfer system the respective principal amounts of the individual debt securities represented by such global security to the accounts of persons that have accounts with such depository ("participants"). Such accounts will be designated by the underwriters, dealers or agents with respect to such debt securities or by us if such debt securities are offered directly by us. Ownership of beneficial interests in such global security will be limited to participants or persons that may hold interests through participants.

We expect that, pursuant to procedures established by DTC, ownership of beneficial interests in any global security with respect to which DTC is the depository will be shown on, and the transfer of that ownership will be effected only through, records maintained by DTC or its nominee (with respect to beneficial interests of participants) and records of participants (with respect to beneficial interests of

persons who hold through participants). Neither we nor the trustee will have any responsibility or liability for any aspect of the records of DTC or for maintaining, supervising or reviewing any records of DTC or any of its participants relating to beneficial ownership interests in the debt securities. The laws of some states require that certain purchasers of securities take physical delivery of such securities in definitive form. Such limits and laws may impair the ability to own, pledge or transfer beneficial interest in a global security.

So long as the depository for a global security or its nominee is the registered owner of such global security, such depository or such nominee, as the case may be, will be considered the sole owner or holder of the debt securities represented by such global security for all purposes under the applicable indenture. Except as described below or in the applicable prospectus supplement, owners of beneficial interest in a global security will not be entitled to have any of the individual debt securities represented by such global security registered in their names, will not receive or be entitled to receive physical delivery of any such debt securities in definitive form and will not be considered the owners or holders thereof under the applicable indenture. Beneficial owners of debt securities evidenced by a global security will not be considered the owners or holders thereof under the applicable indenture for any purpose, including with respect to the giving of any direction, instructions or approvals to the trustee thereunder. Accordingly, each person owning a beneficial interest in a global security with respect to which DTC is the depository must rely on the procedures of DTC and, if such person is not a participant, on the procedures of the participant through which such person owns its interests, to exercise any rights of a holder under the applicable indenture. We understand that, under existing industry practice, if it requests any action of holders or if an owner of a beneficial interest in a global security desires to give or take any action which a holder is entitled to give or take under the applicable indenture, DTC would authorize the participants holding the relevant beneficial interest to give or take such action, and such participants would authorize beneficial owners through such participants to give or take such actions or would otherwise act upon the instructions of beneficial owners holding through them.

Payments of principal of, and any interest on, individual debt securities represented by a global security registered in the name of a depository or its nominee will be made to or at the direction of the depository or its nominee, as the case may be, as the registered owner of the global security under the applicable indenture. Under the terms of the applicable indenture, we and the trustee may treat the persons in whose name debt securities, including a global security, are registered as the owners thereof for the purpose of receiving such payments. Consequently, neither we nor the trustee has or will have any responsibility or liability for the payment of such amounts to beneficial owners of debt securities (including principal and interest). We believe, however, that it is currently the policy of DTC to immediately credit the accounts of relevant participants with such payments, in amounts proportionate to their respective holdings of beneficial interests in the relevant global security as shown on the records of DTC or its nominee. We also expect that payments by participants to owners of beneficial interests in such global security held through such participants will be governed by standing instructions and customary practices, as is the case with securities held for the account of customers in bearer form or registered in street name, and will be the responsibility of such participants. Redemption notices with respect to any debt securities represented by a global security will be sent to the depository or its nominee. If less than all of the debt securities of any series are to be redeemed, we expect the depository to determine the amount of the interest of each participant in such debt securities to be redeemed to be determined by lot. None of us, the trustee, any paying agent or the registrar for such debt securities will have any responsibility or liability for any aspect of the records relating to or payments made on account of beneficial ownership interests in the global security for such debt securities or for maintaining any records with respect thereto.

Neither we nor the trustee will be liable for any delay by the holders of a global security or the depository in identifying the beneficial owners of debt securities and we and the trustee may

conclusively rely on, and will be protected in relying on, instructions from the holder of a global security or the depository for all purposes. The rules applicable to DTC and its participants are on file with the SEC.

If a depository for any debt securities is at any time unwilling, unable or ineligible to continue as depository and a successor depository is not appointed by us within 90 days, we will issue individual debt securities in exchange for the global security representing such debt securities. In addition, we may at any time and in our sole discretion, subject to any limitations described in the prospectus supplement relating to such debt securities, determine not to have any of such debt securities represented by one or more global securities and in such event we will issue individual debt securities in exchange for the global security or securities representing such debt securities. Individual debt securities so issued will be issued in denominations of $1,000 and integral multiples thereof.

All moneys paid by us to a paying agent or a trustee for the payment of the principal of or interest on any debt security which remain unclaimed at the end of two years after such payment has become due and payable will be repaid to us, and the holder of such debt security thereafter may look only to us for payment thereof.

Non-U.S. Currency

If the purchase price of any debt securities is payable in a currency other than U.S. dollars, the specific terms with respect to such debt securities and such foreign currency will be specified in the applicable prospectus supplement.

DESCRIPTION OF ARCH CAPITAL DEPOSITARY SHARES

General

We may, at our option, elect to offer depositary shares, each consisting of one or more, or a specified fraction (to be set forth in the prospectus supplement relating to our common shares or a particular series of preference shares) of a common share or a preference share of a particular series as described below. In the event we elect to do so, depositary receipts evidencing depositary shares will be issued to the public.

The common shares or a class or series of preference shares represented by depositary shares will be deposited under a deposit agreement among us, a depositary selected by us and the holders of the depositary receipts. The depositary will be a bank or trust company having its principal office in the United States and having a combined capital and surplus of at least $50,000,000. Subject to the terms of the deposit agreement, each owner of a depositary share will be entitled, in proportion to the applicable fraction of a common share or preference share represented by such depositary share, to all the rights and preferences of the common shares or preference shares represented thereby (including dividend, voting, redemption and liquidation rights).

The depositary shares will be evidenced by depositary receipts issued pursuant to the deposit agreement. Depositary receipts will be distributed to those persons purchasing the fractional shares of the common shares or related class or series of preference shares in accordance with the terms of the offering described in the related prospectus supplement. If we issue depositary shares, we will file copies of the forms of deposit agreement and depositary receipt as exhibits to the registration statement of which this prospectus forms a part, and the following summary is qualified in its entirety by reference to such exhibits.

Pending the preparation of definitive depositary receipts, the depositary may, upon our written order, issue temporary depositary receipts substantially identical to (and entitling the holders thereof to all the rights pertaining to) the definitive depositary receipts but not in definitive form. Definitive

depositary receipts will be prepared thereafter without unreasonable delay, and temporary depositary receipts will be exchangeable for definitive depositary receipts without charge to the holder thereof.

The following description of the depositary shares sets forth the material terms and provisions of the depositary shares to which any prospectus supplement may relate. The particular terms of the depositary shares offered by any prospectus supplement, and the extent to which the general provisions described below may apply to the offered securities, will be described in the prospectus supplement.

Dividends and Other Distributions

The depositary will distribute all cash dividends or other distributions received in respect of the related common shares or class or series of preference shares to the record holders of depositary shares relating to such common shares or class or series of preference shares in proportion to the number of such depositary shares owned by such holders.

In the event of a distribution other than in cash, the depositary will distribute property received by it to the record holders of depositary shares entitled thereto, unless the depositary determines that it is not feasible to make such distribution, in which case the depositary may, with our approval, sell such property and distribute the net proceeds from such sale to such holders.

Withdrawal of Shares

Upon surrender of the depositary receipts at the corporate trust office of the depositary (unless the related depositary shares have previously been called for redemption), the holder of the depositary shares evidenced thereby is entitled to delivery of the number of whole shares of the related common shares or class or series of preference shares and any money or other property represented by such depositary shares. Holders of depositary shares will be entitled to receive whole shares of the related common shares or class or series of preference shares on the basis set forth in the prospectus supplement for such common shares or class or series of preference shares, but holders of such whole common shares or preference shares will not thereafter be entitled to exchange them for depositary shares. If the depositary receipts delivered by the holder evidence a number of depositary shares in excess of the number of depositary shares representing the number of whole common shares or preference shares to be withdrawn, the depositary will deliver to such holder at the same time a new depositary receipt evidencing such excess number of depositary shares. In no event will fractional common shares or preference shares be delivered upon surrender of depositary receipts to the depositary.

Redemption of Depositary Shares

Whenever we redeem common shares or preference shares held by the depositary, the depositary will redeem as of the same redemption date the number of depositary shares representing shares of common shares or the related class or series of preference shares so redeemed. The redemption price per depositary share will be equal to the applicable fraction of the redemption price per share payable with respect to such class or series of the common shares or preference shares. If less than all the depositary shares are to be redeemed, the depositary shares to be redeemed will be selected by lot or pro rata as may be determined by the depositary.

Voting the Common Shares or Preference Shares

Upon receipt of notice of any meeting at which the holders of the common shares or preference shares are entitled to vote, the depositary will mail the information contained in such notice of meeting to the record holders of the depositary shares relating to such common shares or preference shares. Each record holder of such depositary shares on the record date (which will be the same date as the record date for the common shares or preference shares, as applicable) will be entitled to instruct the

depositary as to the exercise of the voting rights pertaining to the amount of the class or series of preference shares or common shares represented by such holder s depositary shares. The depositary will endeavor, insofar as practicable, to vote the number of the common shares or preference shares represented by such depositary shares in accordance with such instructions, and we will agree to take all action which the depositary deems necessary in order to enable the depositary to do so. The depositary will abstain from voting common shares or preference shares to the extent it does not receive specific instructions from the holders of depositary shares representing such common shares or preference shares.

Amendment and Termination of the Deposit Agreement

The form of depositary receipt evidencing the depositary shares and any provision of the deposit agreement may at any time be amended by agreement between us and the depositary. However, any amendment which materially and adversely alters the rights of the holders of depositary receipts will not be effective unless such amendment has been approved by the holders of depositary receipts representing at least a majority (or, in the case of amendments relating to or affecting rights to receive dividends or distributions or voting or redemption rights, 66²/3%, unless otherwise provided in the related prospectus supplement) of the depositary shares then outstanding. The deposit agreement may be terminated by us or the depositary only if (1) all outstanding depositary shares have been redeemed, (2) there has been a final distribution in respect of the common shares or the related class or series of preference shares in connection with our liquidation, dissolution or winding-up and such distribution has been distributed to the holders of depositary receipts or (3) upon the consent of holders of depositary receipts representing not less than 66²/3% of the depositary shares outstanding.

Charges of Depositary

We will pay all transfer and other taxes and governmental charges arising solely from the existence of the depositary arrangements. We will also pay charges of the depositary in connection with the initial deposit of the related common shares or class or series of preference shares and any redemption of such common shares or preference shares. Holders of depositary receipts will pay all other transfer and other taxes and governmental charges and such other charges as are expressly provided in the deposit agreement to be for their accounts.

The depositary may refuse to effect any transfer of a depositary receipt or any withdrawal of shares of common shares or a class or series of preference shares evidenced thereby until all such taxes and charges with respect to such depositary receipt or such common shares or preference shares are paid by the holders thereof.

Non-U.S. Currency

If the purchase price of any depositary shares is payable in a currency other than U.S. dollars, the specific terms with respect to such depositary shares and such foreign currency will be specified in the applicable prospectus supplement.

Miscellaneous

The depositary will forward all reports and communications from us which are delivered to the depositary and which we are required to furnish to the holders of the common shares or preference shares.

DESCRIPTION OF ARCH CAPITAL WARRANTS TO PURCHASE
COMMON SHARES OR PREFERENCE SHARES

General

We may issue common share warrants or preference share warrants independently or together with any securities offered by any prospectus supplement and such common share warrants or preference share warrants may be attached to or separate from such securities. Each series of share warrants will be issued under a separate warrant agreement to be entered into between us and a bank or trust company, as warrant agent, all as set forth in the applicable prospectus supplement. The warrant agent will act solely as our agent in connection with the certificates representing the share warrants and will not assume any obligation or relationship of agency or trust for or with any holders of share warrant certificates or beneficial owners of share warrants.

The following summaries of certain provisions of the warrant agreement and share warrant certificate are not complete. You should look at the warrant agreement relating to, and the applicable share warrant certificate representing, the applicable series of common share warrants or preference share warrants.

The applicable prospectus supplement may also state that any of the terms set forth herein are inapplicable to such series; provided that the information set forth in such prospectus supplement does not constitute material changes to the information herein such that it alters the nature of the offering or the securities offered. Share warrants for the purchase of common shares or preference shares will be offered and exercisable for U.S. dollars only and will be in registered form only.

Terms

An applicable prospectus supplement will set forth and describe other specific terms regarding each series of share warrants offered hereby, including:

(1): the offering price;
(2): the number of common shares or preference shares purchasable upon exercise of each such common share warrant or preference share warrant and the price at which such number of common shares or preference shares may be purchased upon such exercise;
(3): the date on which the right to exercise such share warrants will commence and the date on which such right will expire; and
(4): any other terms of such share warrants.

Exercise of Share Warrants

Each share warrant will entitle the holder thereof to purchase such common shares or preference shares, as the case may be, at such exercise price as will in each case be set forth in, or calculable from, the prospectus supplement relating to the offered share warrants. After the close of business on the expiration date of each share warrant or such later date to which such expiration date may be extended by us, unexercised share warrants will become void.

Share warrants may be exercised by delivering to the warrant agent payment as provided in the applicable prospectus supplement of the amount required to purchase the common shares or preference shares purchasable upon such exercise, together with certain information set forth on the reverse side of the share warrant certificate. Upon receipt of such payment and the share warrant certificate properly completed and duly executed at the corporate trust office of the warrant agent or any other office indicated in the applicable prospectus supplement, we will, as soon as practicable, issue and deliver the common shares or preference shares purchasable upon such exercise. If fewer than all of the share warrants represented by such share certificate are exercised, a new share warrant certificate will be issued for the remaining amount of share warrants.

Amendments and Supplements to Warrant Agreement

The warrant agreement for a series of share warrants may be amended or supplemented without the consent of the holders of the share warrants issued thereunder to effect changes that are not inconsistent with the provisions of the share warrants and that do not adversely affect the interests of the holders of the share warrants.

Anti-dilution and Other Provisions

Unless otherwise indicated in the applicable prospectus supplement, the exercise price of, and the number of common shares or preference shares covered by, each share warrant is subject to adjustment in certain events, including:

(1): the issuance of common shares or preference shares as a dividend or distribution on the common shares or preference shares;
(2): certain subdivisions and combinations of the common shares or preference shares;
(3): the issuance to all holders of common shares or preference shares of certain rights or warrants entitling them to subscribe for or purchase common shares or preference shares, at less than the current market value, as defined in the applicable warrant agreement for such series of share warrants; and
(4): the distribution to all holders of common shares or preference shares of certain evidences of our indebtedness or assets, other than certain cash dividends and distributions described below.

No adjustment in the exercise price of, and the number of shares covered by, the share warrant will be made for regular quarterly or other periodic or recurring cash dividends or distributions or for cash dividends or distributions to the extent paid from retained earnings. No adjustment will be required unless such adjustment would require a change of at least one percent in the exercise price and exercise rate then in effect; provided, however, that any such adjustment not so made will be carried forward and taken into account in any subsequent adjustment; provided, further, that any such adjustment not so made will be made no later than three years after the occurrence of the event requiring such adjustment to be made or carried forward. Except as stated above, the exercise price of, and the number of common shares or preference shares covered by, a share warrant will not be adjusted for the issuance of common shares or preference shares or any securities convertible into or exchangeable for common shares or preference shares, or securities carrying the right to purchase any of the foregoing.

In the case of:

(1): a reclassification or change of the common shares or preference shares;
(2): certain consolidation or merger events involving us; or
(3): a sale or conveyance to another corporation of our property and assets as an entirety or substantially as an entirety, in each case as a result of which holders of our common shares or preference shares will be entitled to receive stock, securities, other property or assets (including cash) with respect to or in exchange for such shares, the holders of the share warrants then outstanding will be entitled thereafter to convert such share warrants into the kind and amount of common shares, preference shares and other securities or property which they would have received upon such reclassification, change, consolidation, merger, sale or conveyance had such share warrants been exercised immediately prior to such reclassification, change, consolidation, merger, sale or conveyance.

Non-U.S. Currency

If the purchase price of any warrants to purchase common shares or preference shares is payable in a currency other than U.S. dollars, the specific terms with respect to such warrants to purchase common shares or preference shares and such foreign currency will be specified in the applicable prospectus supplement.

DESCRIPTION OF ARCH CAPITAL WARRANTS TO PURCHASE DEBT SECURITIES

General

We may issue debt warrants independently or together with any securities offered by any prospectus supplement and such debt warrants may be attached to or separate from such securities. Each series of debt warrants will be issued under a separate debt warrant agreement to be entered into between us and a debt warrant agent, all as set forth in the applicable prospectus supplement. The debt warrant agent will act solely as our agent in connection with the certificates representing the debt warrants and will not assume any obligation or relationship of agency or trust for or with any holders of debt warrant certificates or beneficial owners of debt warrants.

The following summaries of certain provisions of the debt warrant agreement and debt warrant certificate are not complete. You should look at the debt warrant agreement relating to, and the debt warrant certificate representing, a series of debt warrants.

The applicable prospectus supplement may also state that any of the terms set forth herein are inapplicable to such series; provided, that the information set forth in such prospectus supplement does not constitute material changes to the information herein such that it alters the nature of the offering or the securities offered. Debt warrants for the purchase of common shares or preference shares will be offered and exercisable for U.S. dollars only and will be in registered form only.

Terms

An applicable prospectus supplement will set forth and describe other specific terms regarding each series of debt warrants offered hereby, including:

(1): the offering price;
(2): the designation, aggregate principal amount and the terms of the debt securities purchasable upon exercise of the debt warrants;
(3): the date on which the right to exercise such share warrants will commence and the date on which such right will expire; and
(4): any other terms of such share warrants.

Warrant holders will not have any of the rights of holders of debt securities, including the right to receive the payment of principal of, any premium or interest on, or any additional amounts with respect to, the debt securities or to enforce any of the covenants of the debt securities or the applicable indenture except as otherwise provided in the applicable indenture.

Exercise of Debt Warrants

Debt warrants may be exercised by delivering to the debt warrant agent payment as provided in the applicable prospectus supplement, together with certain information set forth on the reverse side of the debt warrant certificate. Upon receipt of such payment and the debt warrant certificate properly completed and duly executed at the corporate trust office of the debt warrant agent or any other office indicated in the applicable prospectus supplement, we will, as soon as practicable, issue and deliver the debt securities purchasable upon such exercise. If fewer than all of the debt warrants represented by

such debt warrant certificate are exercised, a new debt warrant certificate will be issued for the remaining amount of debt warrants.

Amendments and Supplements to Warrant Agreement

The debt warrant agreement for a series of debt warrants may be amended or supplemented without the consent of the holders of the debt warrants issued thereunder to effect changes that are not inconsistent with the provisions of the debt warrants and that do not adversely affect the interests of the holders of the debt warrants.

Non-U.S. Currency

If the purchase price of any warrants to purchase debt securities is payable in a currency other than U.S. dollars, the specific terms with respect to such warrants to purchase debt securities and such foreign currency will be specified in the applicable prospectus supplement.

DESCRIPTION OF ARCH CAPITAL
SHARE PURCHASE CONTRACTS AND SHARE PURCHASE UNITS

Arch Capital may issue share purchase contracts, representing contracts obligating holders to purchase from Arch Capital, and obligating Arch Capital to sell to the holders, or holders to sell to Arch Capital and Arch Capital to purchase from the holders, a fixed or varying number of common shares at a future date or dates. The price per common share may be fixed at the time the share purchase contracts are entered into or may be determined by reference to a specific formula set forth in the share purchase contracts. Any share purchase contract may include anti-dilution provisions to adjust the number of shares to be delivered pursuant to such share purchase contract upon the occurrence of certain events. The share purchase contracts may be entered into separately or as a part of share purchase units consisting of one or more share purchase contracts and any one or more of:

(1): debt securities of Arch Capital (which may be senior or subordinated);
(2): senior debt securities of Arch Capital Group (U.S.) Inc., fully and unconditionally guaranteed by Arch Capital;
(3): preference shares of Arch Capital; or
(4): debt or equity obligations of third parties, including U.S. Treasury securities.

The share purchase contracts may require us to make periodic payments to the holders of the share purchase units or vice versa, and such payments may be unsecured or pre-funded and may be paid on a current or on a deferred basis. The share purchase contracts may require holders to secure their obligations in a specified manner and in certain circumstances we may deliver newly issued prepaid share purchase contracts upon release to a holder of any collateral securing such holder's obligations under the original share purchase contract. Any one or more of the above securities, common shares or the share purchase contracts or other collateral may be pledged as security for the holders' obligations to purchase or sell, as the case may be, the common shares under the share purchase contracts. The share purchase contracts may also allow the holders, under certain circumstances, to obtain the release of the security for their obligations under such contracts by depositing with the collateral agent as substitute collateral, treasury securities with a principal amount at maturity equal to the collateral so released or the maximum number of common shares deliverable by such holders under common share purchase contracts requiring the holders to sell common shares to Arch Capital.

The applicable prospectus supplement may contain, where applicable, the following information about the stock purchase contracts and stock purchase units, as the case may be:

•: whether the stock purchase contracts obligate the holder to purchase or sell, or both purchase and see, our common shares and the nature and amount of the common shares, or the method of determining those amounts;
•: whether the stock purchase contracts are to be prepaid or not;
•: whether the stock purchase contracts are to be settled by delivery, or by reference or linkage to the value, performance or level of our common shares;
•: any acceleration, cancellation, termination or other provisions relating to the settlement of the stock purchase contracts;
•: the designation and terms of the units and of the securities composing the units, including whether and under what circumstances those securities may be held or transferred separately;
•: any provisions for the issuance, payment, settlement, transfer or exchange of the units or of the securities comprising the units; and
•: whether the stock purchase contracts and/or stock purchase units will be issued fully registered or global form.

The applicable prospectus supplement will describe the terms of any share purchase contracts or share purchase units and, if applicable, prepaid share purchase contracts. The description in the prospectus supplement will be qualified in its entirety by reference to (1) the share purchase contracts, (2) the collateral arrangements and depositary arrangements, if applicable, relating to such share purchase contracts or share purchase units and (3) if applicable, the prepaid share purchase contracts and the document pursuant to which such prepaid share purchase contracts will be issued.

Non-U.S. Currency

If the purchase price of any share purchase contract is payable in a currency other than U.S. dollars, the specific terms with respect to such share purchase contract and such foreign currency will be specified in the applicable prospectus supplement.

DESCRIPTION OF ARCH CAPITAL GROUP (U.S.) INC. SENIOR DEBT SECURITIES

General

In this section, references to "Arch US," "we," "our" or "us" refer solely to Arch Capital Group (U.S.) Inc.

Arch Capital Group (U.S.) Inc. may issue senior debt securities from time to time in one or more series, under one or more indentures, each dated as of a date on or prior to the issuance of the senior debt securities to which it relates. Senior debt securities may be issued pursuant a senior indenture between us and a trustee qualified under the Trust Indenture Act. The senior indenture, as amended or supplemented from time to time, is sometimes referred to as an "indenture." The indenture will be subject to and governed by the Trust Indenture Act. The form of such indenture has been filed as an exhibit to the registration statement of which this prospectus is a part, subject to such amendments and supplements as may be adopted from time to time. The aggregate principal amount of senior debt securities which may be issued under the indenture will be unlimited and the indenture will set forth the specific terms of any series of senior debt securities or provide that such terms will be set forth in, or determined pursuant to, an authorizing resolution, as defined in the applicable prospectus supplement, and/or a supplemental indenture, if any, relating to such series.

The statements made below relating to the senior debt securities and the indentures are summaries of the anticipated provisions thereof, do not purport to be complete and are subject to, and are qualified in their entirety by reference to, all of the provisions of the indenture and any applicable U.S. federal income tax considerations as well as any applicable modifications of or additions to the general terms described below in the applicable prospectus supplement. The applicable prospectus supplement may also state that any of the terms set forth herein are inapplicable to such series of senior debt securities; provided, that the information set forth in such prospectus supplement does not constitute material changes to the information herein such that it alters the nature of the offering of the securities offered.

Terms

The senior debt securities will be the direct, unsecured and unsubordinated obligations of Arch Capital Group (U.S.) Inc. and will be fully and unconditionally guaranteed by Arch Capital Group Ltd., the guarantor. The senior debt securities will rank equal in right of payment with all of Arch US's other unsecured and unsubordinated indebtedness.

The senior debt securities will rank equal in right of payment with all Arch US's other unsecured and unsubordinated indebtedness.

The specific terms of each series of senior debt securities will be set forth in the applicable prospectus supplement relating thereto, including the following, as applicable:

(1) the title of such senior debt securities;

(2) the aggregate principal amount of such senior debt securities and any limit on such aggregate principal amount;

(3) if the price (expressed as a percentage of the principal amount thereof) at which such senior debt securities will be issued is an amount other than the principal amount thereof, the portion of the principal amount thereof payable upon declaration of acceleration of the maturity thereof, or, if applicable, the portion of the principal amount of such senior debt securities that is convertible into Arch Capital's common shares or Arch Capital's preference shares or the method by which any such portion will be determined;

(4) if convertible into Arch Capital's common shares or Arch Capital's preference shares, the terms on which such senior debt securities are convertible, including the initial conversion price, the conversion period, any events requiring an adjustment of the applicable conversion price and any requirements relating to the reservation of such Arch Capital common shares or Arch Capital preference shares for purposes of conversion;

(5) the date(s), or the method for determining such date or dates, on which the principal of such senior debt securities will be payable and, if applicable, the terms on which such maturity may be extended;

(6) the rate(s) (which may be fixed or floating), or the method by which such rate or rates will be determined, at which such senior debt securities will bear interest, if any, including, if applicable, that such senior debt securities will bear interest at an increased rate (up to a specified maximum) upon the occurrence of an event of default and/or certain circumstances described in the applicable prospectus supplement (which may include, among other things, a reduction in the trading price of Arch Capital's common shares below certain levels for a minimum period of time);

whom such interest will be payable, and the basis upon which interest will be calculated if other than that of a 360-day year of twelve 30-day months;

(8) the place(s) where the principal of and interest, if any, on such senior debt securities will be payable, where such senior debt securities may be surrendered for registration of transfer or exchange and where notices or demands to or upon Arch US in respect of such senior debt securities and the applicable indenture may be served;

(9) the period(s), if any, within which, the price or prices at which and the other terms and conditions upon which such senior debt securities may, pursuant to any optional or mandatory redemption provisions, be redeemed, as a whole or in part, at Arch US's option;

(10) Arch US's obligation, if any, to redeem, repay or purchase such senior debt securities pursuant to any sinking fund (as defined in the indenture) or analogous provision or at the option of a holder thereof, and the period or periods within which, the price or prices at which and the other terms and conditions upon which such senior debt securities will be redeemed, repaid or purchased, as a whole or in part, pursuant to such obligations;

(11) if other than U.S. dollars, the currency or currencies in which the principal of and interest, if any, on such senior debt securities are denominated and payable, which may be a foreign currency or units of two or more foreign currencies or a composite currency or currencies, and the terms and conditions relating thereto;

(12) whether the amount of payments of principal of or interest, if any, on such senior debt securities may be determined with reference to an index, formula or other method (which index, formula or method may, but need not be, based on the yield on or trading price of other securities, including United States Treasury securities, or on a currency, currencies, currency unit or units, or composite currency or currencies) and the manner in which such amounts will be determined;

(13) whether the principal of or interest, if any, on the senior debt securities of the series are to be payable, at our election or a holder thereof, in a currency or currencies, currency unit or units or composite currency or currencies other than that in which such senior debt securities are denominated or stated to be payable and the period or periods within which, and the terms and conditions upon which, such election may be made;

(14) provisions, if any, granting special rights to the holders of senior debt securities of the series upon the occurrence of such events as may be specified;

(15) any deletions from, modifications of or additions to the events of default or our covenants with respect to senior debt securities of the series, whether or not such events of default or covenants are consistent with the events of default or covenants described herein;

(16) whether senior debt securities of the series are to be issuable initially in temporary global form and whether any senior debt securities of the series are to be issuable in permanent global form and, if so, whether beneficial owners of interests in any such security in permanent global form may exchange such interests for senior debt securities of such series and of like tenor of any authorized form and denomination and the circumstances under which any such exchanges may occur, if other than in the manner provided in the indenture, and, if senior debt securities of the series are to be issuable as a global security, the identity of the depositary for such series;

(17) the applicability, if any, of the defeasance and covenant defeasance provisions of the applicable indenture to the senior debt securities of the series; and

(18) any other terms of the series of senior debt securities and any additions, deletions or modifications to the indenture.

The senior debt securities, if convertible or exchangeable, will not be convertible into or exchangeable for any securities other than Arch Capital's common shares or Arch Capital's preference shares.

If the applicable prospectus supplement provides, the senior debt securities may be issued at a discount below their principal amount and provide for less than the entire principal amount thereof to be payable upon declaration of acceleration of the maturity thereof.

Except as may be set forth in the applicable prospectus supplement, the senior debt securities will not contain any provisions that would limit our ability to incur indebtedness or that would afford holders of senior debt securities protection against transactions involving us, including a highly leveraged transaction involving us or a change of control. The applicable prospectus supplement will contain information with respect to any deletions from, modifications of or additions to the events of default or covenants described below, including any addition of a covenant or other provision providing event risk or similar protection.

Denomination, Interest, Registration and Transfer

Arch US will issue the senior debt securities of each series only in registered form, without coupons, in denominations of $1,000, or in such other currencies or denominations as set forth in the indenture or specified in, or pursuant to, an authorizing resolution and/or supplemental indenture, if any, relating to such series of senior debt securities.

The principal of and interest, if any, on any series of senior debt securities will be payable at the corporate trust office of the trustee, the address of which will be stated in the applicable prospectus supplement. However, at our option, interest payments may be made by check mailed to the address of the person entitled thereto as it appears in the applicable register for such senior debt securities.

Subject to certain limitations imposed upon senior debt securities issued in book-entry form, the senior debt securities of any series:

•: will be exchangeable for any authorized denomination of other senior debt securities of the same series and of a like aggregate principal amount and tenor upon surrender of such senior debt securities at the trustee's corporate trust office or at the office of any registrar designated by us for such purpose; and
•: may be surrendered for registration of transfer or exchange thereof at the corporate trust office of the trustee or at the office of any registrar designated by us for such purpose.

Guarantee

Payment of principal, premium, if any, and interest on the senior debt securities will be fully and unconditionally guaranteed on an unsecured basis by Arch Capital Group Ltd. The guarantee will be a direct obligation of Arch Capital Group Ltd. ranking equally and ratably in right of payment with all other existing and future unsecured and unsubordinated obligations of Arch Capital Group Ltd. other than obligations preferred by law.

As of March 31, 2004 (after giving effect to our offering of $300.0 million aggregate principal amount of our 7.35% Senior Notes due 2034 in May 2004 and the application of the proceeds therefrom), the aggregate amount of Arch Capital's outstanding indebtedness for money borrowed that would rank equally in right of payment to Arch Capital's guarantee of the Arch US senior debt

securities was approximately $300.0 million. As of such date, no indebtedness of Arch Capital for money borrowed would rank senior in right of payment to Arch Capital's guarantee of the Arch US senior debt securities. All obligations (including insurance obligations of our subsidiaries would be effectively senior to any senior or subordinated debt issued by Arch Capital or Arch US. As of March 31, 2004, the consolidated obligations of our subsidiaries reflected on our balance sheet was approximately $4.3 billion.

Certain Covenants

The applicable prospectus supplement will describe any material covenants in respect of a series of senior debt securities that are not described in this prospectus.

Unless otherwise indicated in a prospectus supplement, senior debt securities will include the provisions described below.

Merger, Amalgamation, Consolidation or Sale of Assets

Arch Capital may not, and will not permit us to, consolidate or amalgamate with or merge with or into any other person (other than a subsidiary) or convey, transfer, sell or lease our properties and assets substantially as an entirety to any person (other than a subsidiary), or permit any person (other than a subsidiary) to consolidate with or merge into us or Arch Capital, as the case may be, unless:

•: either (a) we or Arch Capital, as the case may be, shall be the surviving person or (b) the surviving person (if other than us) shall (1) be a corporation or limited liability company organized and existing under the laws of the United States of America, any state thereof, the District of Columbia, Bermuda, the Cayman Islands or any country or state which is a member of the Organization of Economic Cooperation and Development and (2) expressly assume, by an indenture supplemental to the indenture, executed and delivered to the trustee, in form reasonably satisfactory to the trustee, all of our obligations under the senior debt securities and the indenture;
•: immediately after giving effect to such transaction, no event of default, and no event that, after notice or lapse of time or both, would become an event of default, shall have occurred and be continuing; and
•: we or Arch Capital, as the case may be, shall have delivered to the trustee an officers' certificate stating that such consolidation, merger, conveyance, transfer, sale or lease and, if a supplemental indenture is required in connection with such transaction, such supplemental indenture, comply with the base indenture and that all conditions precedent herein provided for relating to such transaction have been satisfied.

The surviving person of such transaction shall succeed to, and be substituted for, and may exercise every right and power of, us or Arch Capital, as the case may be, under the indenture with the same effect as if such successor had been named as us thereunder; and thereafter, we shall be discharged from all obligations and covenants under the base indenture and the senior debt securities or the guarantee, as the case may be.

Payment of Principal, Premium and Interest

We will duly and punctually pay the principal of (and premium, if any) and interest on the senior debt securities in accordance with their terms.

Maintenance of Office or Agency

We and Arch Capital will maintain an office or agency where senior debt securities may be presented or surrendered for payment and may be surrendered for registration of transfer or exchange and where notices and demands to or upon us or Arch Capital in respect of the senior debt securities and the indenture may be served.

Money for Securities; Payments to Be Held in Trust

If Arch US will at any time act as its own paying agent with respect to any senior debt securities, it will, on or before each due date of the principal of (and premium, if any) or interest on such senior debt securities, segregate and hold in trust for the benefit of the persons entitled thereto a sum sufficient to pay the principal (and premium, if any) or interest so becoming due until such sums will be paid to such persons or otherwise disposed of as herein provided and will promptly notify the trustee of its action or failure so to act.

Corporate Existence

Except as otherwise permitted under "—Merger, Amalgamation, Consolidate or Sale of Assets" above, each of Arch US and Arch Capital will do or cause to be done all things necessary to preserve and keep in full force and effect its corporate existence and material rights (charter and statutory) and franchises; provided, however, that Arch US and Arch Capital will not be required to preserve any such right or franchise if its respective boards of directors or senior management determine that the preservation thereof is no longer desirable in the conduct of the business of Arch US or Arch Capital and that the loss thereof is not disadvantageous in any material respect to the holders of senior debt securities.

Statement by Officers as to Default

Arch US and Arch Capital will each deliver to the trustee, within 120 days after the end of each fiscal year ending after the date hereof, a certificate of the principal executive officer, principal financial officer or principal accounting officer of each Arch US and Arch Capital stating whether or not to the best knowledge of the signer thereof Arch US or Arch Capital is in default in the performance and observance of any of the terms, provisions and conditions of the indenture, and if Arch US or the Arch Capital will be in default, specifying all such defaults and the nature and status thereof of which they may have knowledge.

Waiver of Certain Covenants

Arch US or Arch Capital, as the case may be, may omit in any particular instance to comply with any term, provision or condition set forth in the covenants described above under the captions "—Corporate Existence" and "—Statement by Officers as to Default" if before or after the time for such compliance the holders of at least a majority in principal amount of the outstanding senior debt securities affected by the omission will, by act of such holders, either waive such compliance in such instance or generally waive compliance with such term, provision or condition except to the extent so expressly waived, and, until such waiver will become effective, the obligations of Arch US and Arch Capital and the duties of the trustee in respect of any such term, provision or condition will remain in full force and effect.

Ranking of Senior Debt Securities

The senior debt securities will be the direct, unsecured and unsubordinated obligations of Arch Capital Group (U.S.) Inc. and will be fully and unconditionally guaranteed by Arch Capital, the

guarantor. The senior debt securities will rank equal in right of payment with all of Arch US's other unsecured and unsubordinated indebtedness.

Arch Capital currently conducts substantially all of its operations through its subsidiaries and its subsidiaries generate substantially all of its operating income and cash flow. As a result, distributions and advances from its subsidiaries will be the principal source of funds necessary to meet its debt service obligations. Contractual provisions or laws, as well as its subsidiaries' financial condition and operating and regulatory requirements, may limit its ability to obtain cash from its subsidiaries that it requires to pay our debt service obligations. In addition, because we are a holding company, holders of the senior debt securities will have a junior position to the claims of creditors of Arch Capital's subsidiaries on their assets and earnings.

Discharge and Defeasance

Under the terms of the applicable senior indenture, Arch US will be discharged from any and all obligations in respect of the senior debt securities of any series and Arch Capital will be discharged from any and all obligations in respect of the guarantees of the senior debt securities (except in each case for certain obligations to register the transfer or exchange of senior debt securities, replace stolen, lost or mutilated senior debt securities, maintain paying agencies and hold moneys for payment in trust) if:

(1) Arch US delivers all outstanding debt securities of such series to the trustee for cancellation and pay all sums payable by us under such senior debt securities and the indenture with respect to such series; or

(2) such senior debt securities either have become due and payable or will become due and payable within one year (or scheduled for redemption within one year) and Arch US or Arch Capital deposits with the senior debt securities trustee, in trust:

(a) in the case of any senior debt securities of any series denominated in U.S. dollars, cash or U.S. government obligations sufficient to pay all principal of and interest and premium, if any, on such senior debt securities; and

(b) in the case of any senior debt securities of any series denominated in any currency other than U.S. dollars, an amount of the applicable currency in which such senior debt securities are denominated sufficient to pay all principal of and interest and premium, if any, on such senior debt securities.

In addition, unless the prospectus supplement and supplemental indenture provide otherwise, we and Arch Capital may elect either (1) to defease and be discharged from any and all obligations with respect to such senior debt securities and the guarantee by Arch Capital ("defeasance") or (2) to be released from our and its obligations with respect to such senior debt securities under certain covenants, and any omission to comply with such obligations will not constitute a default or an event of default with respect to such senior debt securities ("covenant defeasance"):

(1) by delivering all outstanding senior debt securities of such series to the trustee for cancellation and paying all sums payable by it under such senior debt securities and the indenture with respect to such series; and

(2) after giving notice to the trustee of our intention to defease all of the senior debt securities of such series, by irrevocably depositing with the trustee or a paying agent

(a) in the case of any senior debt securities of any series denominated in U.S. dollars, cash or U.S. government obligations sufficient to pay all principal of and interest on such senior debt securities; and

(b) in the case of any senior debt securities of any series denominated in any currency other than U.S. dollars, an amount of the applicable currency in which the senior debt securities are denominated sufficient to pay all principal of and interest on such senior debt securities.

Such a trust may only be established if, among other things:

(1) the applicable defeasance or covenant defeasance does not result in a breach or violation of, or constitute a default under, any material agreement or instrument to which we or Arch Capital, as the case may be, is a party or by which we or Arch Capital is bound;

(2) no event of default or event which with notice or lapse of time or both would become an event of default with respect to the senior debt securities to be defeased will have occurred and be continuing on the date of establishment of such a trust after giving effect to such establishment; and

(3) we and Arch Capital have delivered to the trustee an opinion of counsel (as specified in the applicable supplemental indenture) to the effect that the holders will not recognize income, gain or loss for United States federal income tax purposes as a result of such defeasance or covenant defeasance and will be subject to United States federal income tax on the same amounts, in the same manner and at the same times as would have been the case if such defeasance or covenant defeasance had not occurred, and such opinion of counsel, in the case of defeasance, must refer to and be based upon a letter ruling of the Internal Revenue Service received by us and Arch Capital, a Revenue Ruling published by the Internal Revenue Service or a change in applicable United States federal income tax law occurring after the date of the applicable supplemental indenture.

In the event we or Arch Capital effects covenant defeasance with respect to any senior debt securities and such senior debt securities are declared due and payable because of the occurrence of any event of default, other than an event of default with respect to any covenant as to which there has been covenant defeasance, the government obligations on deposit with the trustee will be sufficient to pay amounts due on such senior debt securities at the time of the stated maturity but may not be sufficient to pay amounts due on such senior debt securities at the time of the acceleration resulting from such event of default.

Modification and Waiver

Arch US and Arch Capital, when authorized by a board resolution, and the trustee may modify, amend and/or supplement the applicable indenture and the terms of the applicable series of senior debt securities and the related guarantees by Arch Capital with the consent of the holders of not less than a majority in principal amount of the outstanding senior debt securities of all series affected thereby (voting as a single class); provided, however, without the consent of each holder of the debt securities affected thereby, such amendment, modification or supplement may not (with respect to any debt securities held by a non-consenting holder):

(1) change the stated maturity of the principal of or any installment of interest with respect to the senior debt securities;

(2) reduce the principal amount of, or the rate (or modify the calculation of such principal amount or rate) of interest on, or any additional amounts with respect to, or any premium payable upon the redemption of any senior debt securities;

(3) change the currency of payment of principal of or interest on the senior debt securities;

(4) change the redemption provisions of any securities in any manner adverse to the holders of such securities;

(5) impair the right to institute suit for the enforcement of any payment on or with respect to the senior debt securities;

(6) reduce the above-stated percentage of holders of the senior debt securities of any series necessary to modify or amend the indenture relating to such series;

(7) modify the foregoing requirements or reduce the percentage of outstanding senior debt securities necessary to waive any covenant or past default;

(8) waive certain covenants of the senior debt securities except to increase any percentage vote required or to provide that other provisions of such indenture cannot be modified or waived without the consent of the holders of any senior debt securities offered thereby;

(9) in the case of any convertible senior debt securities, adversely affect the right of a holder to convert the senior debt securities other than as provided in or under the applicable indenture;

(10) release Arch Capital from any of its obligations under the applicable indenture or the related guarantees otherwise than in accordance with the terms of the applicable indenture;

(11) modify or change any provision of the indenture or the related definitions affecting the ranking of the senior debt securities or the related guarantees in a manner which adversely affects the holders of the senior debt securities; or

(12) change the obligation to pay additional amounts with respect to any senior debt security.

Holders of not less than a majority in principal amount of the outstanding senior debt securities of all series affected thereby (voting as a single class) may waive certain past defaults and may waive compliance by us and Arch Capital with any provision of the indenture relating to such senior debt securities (subject to the immediately preceding sentence); provided, however, that without the consent of each holder of senior debt securities affected thereby, no waiver may be made of a default:

(1) in the payment of the principal of or interest on any debt security without the consent of each holder of senior debt securities affected thereby; or

(2) in respect of a covenant or provision of the indenture that cannot be modified or amended without the consent of each holder of the senior debt securities affected.

We, Arch Capital and the trustee may amend or supplement the indentures or waive any provision of such indentures and the senior debt securities without the consent of any holders of senior debt securities in some circumstance, including:

•: to cure any ambiguity, omission, defect or inconsistency;
•: to make any other change that does not, in the good faith opinion of the board of directors of us, Arch Capital and the trustee, adversely affect the interests of holders of such senior debt securities in any material respect;
•: to provide for the assumption of our or Arch Capital's obligations under the applicable indenture by a successor upon any merger, consolidation or asset transfer permitted under the applicable indenture;

•: to provide any security for or additional guarantees of such senior debt securities;
•: to add events of default with respect to such senior debt securities;
•: to add covenants of ours or Arch Capital that would benefit the holders of such senior debt securities or to surrender any rights or powers we or Arch Capital has under the applicable indenture;
•: to make any change necessary for the registration of the senior debt securities and the guarantee under the Securities Act or to comply with the Trust Indenture Act of 1939, or any amendment thereto, or to comply with any requirement of the SEC in connection with the qualification of the applicable indenture under the Trust Indenture Act of 1939; provided, however, that such modification or amendment does not, in the good faith opinion of our or Arch Capital's board of directors and the trustee, adversely affect the interests of the holders of such senior debt securities in any material respect;
•: to provide for uncertificated senior debt securities and guarantees in addition to or in place of certificated senior debt securities and guarantees or to provide for bearer senior securities and guarantees;
•: to add to or change any of the provisions of the applicable indenture to such extent as will be necessary to permit or facilitate the issuance of the senior debt securities in bearer form, registrable or not registrable as to principal, and with or without interest coupons;
•: to change or eliminate any of the provisions of the applicable indenture, provided, however, that any such change or elimination will become effective only when there is no debt security outstanding of any series created prior to the execution of such supplemental indenture which is entitled to the benefit of such provision;
•: to establish the form or terms of senior debt securities of any series or the related guarantees as permitted by the applicable indenture; or
•: to evidence and provide for the acceptance of appointment by a successor trustee with respect to the senior debt securities of one or more series and to add to or change any of the provisions of the applicable indenture as will be necessary to provide for or facilitate the administration of the trusts under the applicable indenture by more than one trustee, pursuant to the requirements of the applicable indenture.

Events of Default and Notice Thereof

The following are events that we anticipate will constitute "events of default" with respect to any series of senior debt securities issued thereunder:

(1) default in the payment of any interest upon any senior debt securities of that series when it becomes due and payable, and continuance of such default for a period of 30 days; or

(2) default in the payment of the principal of (or premium, if any, on) any senior debt securities of that series when due; or

(3) default in the deposit of any sinking fund payment, when and as due by the terms of any senior debt securities of that series; or

(4) default in the performance, or breach, of any covenants of Arch US or Arch Capital (other than those set forth in clause (1) or (2) above) in the indenture, any senior debt securities or the guarantee (other than a covenant or warranty added to the indenture solely for the benefit of another series of senior debt securities) for a period of 60 days after there has been given, and continuance of such by registered or certified mail, to Arch US or Arch Capital by the trustee or

to Arch US, Arch Capital and the trustee by the holders of at least 25% in principal amount of the outstanding senior debt securities a written notice specifying such default or breach and requiring it to be remedied and stating that such notice is a "Notice of Default" under the indenture; or

(5) certain events of bankruptcy, insolvency or reorganization; or

(6) the guarantee ceases to be in full force and effect or is declared to be null and void and unenforceable or is found to be invalid, in each case by a court of competent jurisdiction in a final non-appealable judgment, or the guarantor denies its liability under the guarantee (other than by reason of release of the guarantor in accordance with the terms of the indenture).

Additional or different events of default, if any, applicable to the series of senior debt securities in respect of which this prospectus is being delivered will be specified in the applicable prospectus supplement, and if necessary, a post-effective amendment to this registration statement.

The trustee under such indenture will, within 90 days after the occurrence of any default (the term "default" to include the events specified above without grace or notice) with respect to any series of senior debt securities actually known to it, give to the holders of such senior debt securities notice of such default; provided, however, that, except in the case of a default in the payment of principal of or interest on any of the senior debt securities of such series or in the payment of a sinking fund installment, the trustee for such series will be protected in withholding such notice if it in good faith determines that the withholding of such notice is in the interest of the holders of such senior debt securities; and provided, further, that in the case of any default of the character specified in clause (2) above with respect to senior debt securities of such series, no such notice to holders of such senior debt securities will be given until at least 30 days after the occurrence thereof. We will certify to the trustee within 120 days after the end of each fiscal year as to whether any default exists.

In the case of an event of default, other than an event of default resulting from bankruptcy, insolvency or reorganization, with respect to any series of senior debt securities will occur and be continuing, the trustee for such series or the holders of at least 25% in aggregate principal amount of the senior debt securities of such series then outstanding, by notice in writing to us (and to the trustee for such series if given by the holders of the senior debt securities of such series), will be entitled to declare all unpaid principal of and accrued interest on such senior debt securities then outstanding to be due and payable immediately.

In the case of an event of default resulting from certain events of bankruptcy, insolvency or reorganization, all unpaid principal of and accrued interest on all senior debt securities of such series then outstanding will be due and payable immediately without any declaration or other act on the part of the trustee for such series or the holders of any senior debt securities of such series.

Such acceleration may be annulled and past defaults (except, unless theretofore cured, a default in payment of principal of or interest on the senior debt securities of such series) may be waived by the holders of a majority in principal amount of the senior debt securities of such series then outstanding upon the conditions provided in the applicable indenture.

No holder of the senior debt securities of any series issued thereunder may pursue any remedy under such indenture unless the trustee for such series will have failed to act after, among other things, notice of an event of default and request by holders of at least 25% in principal amount of the senior debt securities of such series of which the event of default has occurred and the offer to the trustee for such series of indemnity satisfactory to it; provided, however, that such provision does not affect the right to sue for enforcement of any overdue payment on such senior debt securities.

Conversion and Exchange Rights

The terms and conditions, if any, upon which the senior debt securities of any series will be convertible into Arch Capital's common shares or Arch Capital's preference shares or upon which the senior debt securities of any series will be exchangeable into another series of senior debt securities will be set forth in the prospectus supplement relating thereto. Such terms will include the conversion or exchange price (or manner of calculation thereof), the conversion or exchange period, provisions as to whether conversion or exchange will be at the option of the holders of such series of senior debt securities or at our option or automatic, the events requiring an adjustment of the conversion or exchange price and provisions affecting conversion or exchange in the event of the redemption of such series of senior debt securities.

The Trustee

The trustee for each series of senior debt securities will be named in the prospectus supplement relating to each issuance of senior debt securities. The indenture will contain certain limitations on a right of the trustee, as our creditor, to obtain payment of claims in certain cases, or to realize on certain property received in respect of any such claim as security or otherwise. The trustee will be permitted to engage in other transactions; provided, however, that if it acquires any conflicting interest, it must eliminate such conflict or resign.

Subject to the terms of the applicable indenture, the holders of a majority in principal amount of all outstanding senior debt securities of a series (or if more than one series is affected thereby, of all series so affected, voting as a single class) will have the right to direct the time, method and place of conducting any proceeding for exercising any remedy or power available to the trustee for such series or all such series so affected.

In case an event of default will occur (and will not be cured) under any indenture relating to a series of senior debt securities and is actually known to a responsible officer of the trustee for such series, such trustee will exercise such of the rights and powers vested in it by such indenture and use the same degree of care and skill in its exercise as a prudent person would exercise or use under the circumstances in the conduct of his own affairs. Subject to such provisions, the trustee will not be under any obligation to exercise any of its rights or powers under the applicable indenture at the request of any of the holders of senior debt securities unless they will have offered to the trustee security and indemnity satisfactory to it.

Governing Law

The indentures and the senior debt securities will be governed by the laws of the State of New York.

Global Securities; Book-Entry System

We may issue the senior debt securities of any series and the related guarantees in whole or in part in the form of one or more global securities to be deposited with, or on behalf of, a depositary identified in the prospectus supplement relating to such series. "Global securities" represent in the aggregate the total principal or face amount of the securities and once on deposit with a depositary, allow trading of the securities through the depositary's book-entry system (as further described below). Global securities, if any, issued in the United States are expected to be deposited with DTC, as depositary. Global securities will be issued in fully registered form and may be issued in either temporary or permanent form. Unless and until it is exchanged in whole or in part for the individual senior debt securities represented thereby, a global security may not be transferred except as a whole by the depositary for such global security to a nominee of such depositary or by a nominee of such

depositary to such depositary or another nominee of such depositary or by such depositary or any nominee of such depositary to a successor depositary or any nominee of such successor.

The specific terms of the depositary arrangement with respect to any series of senior debt securities will be described in the prospectus supplement relating to such series. We expect that unless otherwise indicated in the applicable prospectus supplement, the following provisions will apply to depositary arrangements.

Upon the issuance of a global security, the depositary for such global security or its nominee will credit on its book-entry registration and transfer system the respective principal amounts of the individual senior debt securities represented by such global security to the accounts of participants. Such accounts will be designated by the underwriters, dealers or agents with respect to such senior debt securities or by us if such senior debt securities are offered directly by us. Ownership of beneficial interests in such global security will be limited to participants or persons that may hold interests through participants.

We expect that, pursuant to procedures established by DTC, ownership of beneficial interests in any global security with respect to which DTC is the depositary will be shown on, and the transfer of that ownership will be effected only through, records maintained by DTC or its nominee (with respect to beneficial interests of participants) and records of participants (with respect to beneficial interests of persons who hold through participants). None of us, Arch Capital or the trustee will have any responsibility or liability for any aspect of the records of DTC or for maintaining, supervising or reviewing any records of DTC or any of its participants relating to beneficial ownership interests in the senior debt securities. The laws of some states require that certain purchasers of securities take physical delivery of such securities in definitive form. Such limits and laws may impair the ability to own, pledge or transfer beneficial interest in a global security.

So long as the depositary for a global security or its nominee is the registered owner of such global security, such depositary or such nominee, as the case may be, will be considered the sole owner or holder of the senior debt securities represented by such global security for all purposes under the applicable indenture. Except as described below or in the applicable prospectus supplement, owners of beneficial interest in a global security will not be entitled to have any of the individual senior debt securities represented by such global security registered in their names, will not receive or be entitled to receive physical delivery of any such senior debt securities in definitive form and will not be considered the owners or holders thereof under the applicable indenture. Beneficial owners of senior debt securities evidenced by a global security will not be considered the owners or holders thereof under the applicable indenture for any purpose, including with respect to the giving of any direction, instructions or approvals to the trustee thereunder. Accordingly, each person owning a beneficial interest in a global security with respect to which DTC is the depositary must rely on the procedures of DTC and, if such person is not a participant, on the procedures of the participant through which such person owns its interests, to exercise any rights of a holder under the applicable indenture. We understand that, under existing industry practice, if it requests any action of holders or if an owner of a beneficial interest in a global security desires to give or take any action which a holder is entitled to give or take under the applicable indenture, DTC would authorize the participants holding the relevant beneficial interest to give or take such action, and such participants would authorize beneficial owners through such participants to give or take such actions or would otherwise act upon the instructions of beneficial owners holding through them.

Payments of principal of, and any interest on, individual senior debt securities represented by a global security registered in the name of a depositary or its nominee will be made to or at the direction of the depositary or its nominee, as the case may be, as the registered owner of the global security under the applicable indenture. Under the terms of the applicable indenture, we and the trustee may treat the persons in whose name senior debt securities, including a global security, are registered as the

owners thereof for the purpose of receiving such payments. Consequently, neither we nor the trustee has or will have any responsibility or liability for the payment of such amounts to beneficial owners of senior debt securities (including principal and interest). We believe, however, that it is currently the policy of DTC to immediately credit the accounts of relevant participants with such payments, in amounts proportionate to their respective holdings of beneficial interests in the relevant global security as shown on the records of DTC or its nominee. We also expect that payments by participants to owners of beneficial interests in such global security held through such participants will be governed by standing instructions and customary practices, as is the case with securities held for the account of customers in bearer form or registered in street name, and will be the responsibility of such participants. Redemption notices with respect to any senior debt securities represented by a global security will be sent to the depositary or its nominee. If less than all of the senior debt securities of any series are to be redeemed, we expect the depositary to determine the amount of the interest of each participant in such senior debt securities to be redeemed to be determined by lot. None of us, Arch Capital or the trustee, any paying agent or the registrar for such senior debt securities will have any responsibility or liability for any aspect of the records relating to or payments made on account of beneficial ownership interests in the global security for such senior debt securities or for maintaining any records with respect thereto.

None of us, Arch Capital or the trustee will be liable for any delay by the holders of a global security or the depositary in identifying the beneficial owners of senior debt securities and we and the trustee may conclusively rely on, and will be protected in relying on, instructions from the holder of a global security or the depositary for all purposes. The rules applicable to DTC and its participants are on file with the SEC.

If a depositary for any senior debt securities is at any time unwilling, unable or ineligible to continue as depositary and a successor depositary is not appointed by us within 90 days, we will issue individual senior debt securities in exchange for the global security representing such senior debt securities. In addition, we may at any time and in our sole discretion, subject to any limitations described in the prospectus supplement relating to such senior debt securities, determine not to have any of such senior debt securities represented by one or more global securities and in such event we will issue individual senior debt securities in exchange for the global security or securities representing such senior debt securities. Individual senior debt securities so issued will be issued in denominations of $1,000 and integral multiples thereof.

All moneys paid by us or Arch Capital to a paying agent or a trustee for the payment of the principal of or interest on any debt security which remain unclaimed at the end of two years after such payment has become due and payable will be repaid to us or Arch Capital, as the case may be, and the holder of such debt security thereafter may look only to us for payment thereof.

Non-U.S. Currency

If the purchase price of any Arch US senior debt securities is payable in a currency other than U.S. dollars, the specific terms with respect to such Arch US debt securities and such foreign currency will be specified in the applicable prospectus supplement.

PLAN OF DISTRIBUTION

Arch Capital may sell its common shares, preference shares, debt securities, depositary shares, warrants to purchase common shares, preference shares or debt securities, share purchase contracts and share purchase units and Arch US may sell its senior debt securities, through underwriters, agents, dealers, or directly without the use of any underwriter, agent or dealer to one or more purchasers. Arch Capital and Arch US may distribute these securities from time to time in one or more transactions, including, but not limited to, block transactions, privately negotiated transactions,

transactions on The Nasdaq National Market or any other organized market where the securities may be traded, through the writing of options on securities, short sales or any combination of these methods. The securities may be sold at a fixed price or prices, at market prices prevailing at the times of sale, at prices related to these prevailing market prices or at negotiated prices. Any such price may be changed from time to time.

We may sell equity securities in an offering "at the market" as defined in Rule 415 under the Securities Act. A post-effective amendment to this registration statement will be filed to identify the underwriter(s) at the time of the take-down for "at the market" offerings.

In addition, we may enter into derivative transactions with third parties (including the writing of options), or sell securities not covered by this prospectus to third parties in privately negotiated transactions. If the applicable prospectus supplement indicates, in connection with such a transaction the third parties may, pursuant to this prospectus and the applicable prospectus supplement, sell securities covered by this prospectus and the applicable prospectus supplement. If so, the third party may use securities borrowed from us or others to settle such sales and may use securities received from us to close out any related short positions. We may also loan or pledge securities covered by this prospectus and the applicable prospectus supplement to third parties, who may sell the loaned securities or, in an event of default in the case of a pledge, sell the pledged securities pursuant to this prospectus and the applicable prospectus supplement.

The terms of the offering of the securities with respect to which this prospectus is being delivered will be set forth in the applicable prospectus supplement and will include:

•: the identity of any underwriters, dealers or agents who purchase securities, as required;
•: the amount of securities sold, the public offering price and consideration paid, and the proceeds Arch Capital and/or Arch US will receive from that sale;
•: whether or not the securities will trade on any securities exchanges or The Nasdaq National Market;
•: the amount of any indemnification provisions, including indemnification from liabilities under the federal securities laws; and
•: any other material terms of the distribution of securities.

Arch Capital and/or Arch US may offer the securities to the public through one or more underwriting syndicates represented by one or more managing underwriters, or through one or more underwriters without a syndicate. If underwriters are used in the sale, Arch Capital and/or Arch US will execute an underwriting agreement with those underwriters relating to the securities that Arch Capital and/or Arch US will offer and will name the underwriters and describe the terms of the transaction in the prospectus supplement. The securities subject to the underwriting agreement will be acquired by the underwriters for their own account and may be resold by them, or their donees, pledgees, or transferees, from time to time in one or more transactions, including negotiated transactions, at a fixed public offering price or at varying prices determined at the time of sale. Subject to the conditions specified in the underwriting agreement, underwriters will be obligated to purchase all of these securities if they are purchased or will act on a best efforts basis to solicit purchases for the period of their appointment, unless stated otherwise in the prospectus supplement.

Arch Capital and/or Arch US may authorize underwriters to solicit offers by institutions to purchase the securities subject to the underwriting agreement from Arch Capital and/or Arch US at the public offering price stated in the prospectus supplement under delayed delivery contracts providing for payment and delivery on a specified date in the future. If Arch Capital and/or Arch US sell securities under delayed delivery contracts, the prospectus supplement will state that as well as the conditions to which these delayed delivery contracts will be subject and the commissions payable for that solicitation.

Underwriters may sell these securities to or through dealers. Alternatively Arch Capital and/or Arch US may sell the securities in this offering to one or more dealers, who would act as a principal or principals. Dealers may resell such securities to the public at varying prices to be determined by the dealers at the time of the resale.

Arch Capital and/or Arch US may also sell the securities offered with this prospectus through other agents designated by them from time to time. Arch Capital and Arch US will identify any agent involved in the offer and sale of these securities who may be deemed to be an underwriter under the federal securities laws, and describe any commissions or discounts payable by Arch Capital and/or Arch US to these agents, in the prospectus supplement. Any such agents will be obligated to purchase all of these securities if any are purchased or will act on a best efforts basis to solicit purchases for the period of their appointment, unless stated otherwise in the prospectus supplement.

In connection with the sale of securities offered under this prospectus, underwriters, dealers or agents may receive compensation from Arch Capital, Arch US or from purchasers of the securities for whom they may act as agents, in the form of discounts, concessions or commissions. These discounts, concessions or commissions may be changed from time to time. Underwriters, dealers and/or agents may engage in transactions with Arch Capital or Arch US, or perform services for Arch Capital or Arch US, in the ordinary course of business, and may receive compensation in connection with those arrangements. In the event any underwriter, dealer or agent who is a member of the NASD participates in a public offering of these securities, the maximum commission or discount to be received by any such NASD member or independent broker-dealer will not be greater than 8% of the offering proceeds from securities offered with this prospectus.

Underwriters, dealers, agents or purchasers that participate in the distribution of the securities may be deemed to be underwriters under the Securities Act. Broker-dealers or other persons acting on behalf of parties that participate in the distribution of securities may also be deemed underwriters. Any discounts or commissions received by them and any profit on the resale of the securities received by them may be deemed to be underwriting discounts and commissions under the Securities Act.

Underwriters and purchasers that are deemed underwriters under the Securities Act may engage in transactions that stabilize, maintain or otherwise affect the price of the securities, including the entry of stabilizing bids or syndicate covering transactions or the imposition of penalty bids. Such purchasers will be subject to the applicable provisions of the Securities Act and Exchange Act and the rules and regulations thereunder, including Rule 10b-5 and Regulation M. Regulation M may restrict the ability of any person engaged in the distribution of the securities to engage in market-making activities with respect to those securities. In addition, the anti-manipulation rules under the Exchange Act may apply to sales of the securities in the market. All of the foregoing may affect the marketability of the securities and the ability of any person to engage in market-making activities with respect to the securities.

Arch Capital and/or Arch US may provide underwriters, agents, dealers or purchasers with indemnification against civil liabilities, including liabilities under the Securities Act, or contribution with respect to payments that the underwriters, agents, dealers or purchasers may make with respect to such liabilities.

WHERE YOU CAN FIND MORE INFORMATION

We and Arch US have filed with the SEC under the Securities Act of 1933, as amended, a combined registration statement of Form S-3 relating to the offered securities.

Arch Capital

Arch Capital is subject to the informational requirements of the Securities Exchange Act of 1934, and in accordance therewith files reports, proxy statements and other information with the SEC. These reports, proxy statements and other information contain additional information about Arch Capital. You can inspect and copy these materials at the SEC's Public Reference Room at 100 F Street, N.E., Washington, D.C. 20549. You can obtain information about the Public Reference Room by calling the SEC at 1-800-SEC-0330. The SEC maintains an Internet website that contains reports, proxy and information statements and other information regarding companies that file electronically with the SEC. The SEC's Internet address is http://www.sec.gov. You can also inspect these materials at the offices of the Nasdaq Stock Market, 1735 K Street, N.W., Washington, D.C. 20006.

Arch Capital Group (U.S.) Inc.

Arch Capital Group (U.S.) Inc. is not currently subject to the information reporting requirements of the Exchange Act. Arch Capital Group (U.S.) Inc. is a wholly owned subsidiary of Arch Capital. Any future issuances of debt securities of Arch Capital Group (U.S.) Inc. will be fully and unconditionally guaranteed by Arch Capital and by no other subsidiary of Arch Capital. See "Description of Arch Capital Group (U.S.) Inc. Senior Debt Securities."

INCORPORATION OF DOCUMENTS BY REFERENCE

The SEC allows Arch Capital to "incorporate by reference" information into this prospectus, which means that it can disclose important information by referring you to another document filed separately with the SEC. This prospectus incorporates by reference the documents listed below which have been filed by Arch Capital with the SEC and any future filings with the SEC under Section 13(a), 13(c), 14 or 15(d) of the Exchange Act.

•: Annual report on Form 10-K for the year ended December 31, 2003.
•: Quarterly report on Form 10-Q for the quarter ended March 31, 2004.
•: Current reports on Form 8-K filed on March 24, 2004, April 8, 2004, and May 7, 2004.
•: Proxy statement on Schedule 14A for our annual meeting of shareholders held on May 5, 2004 (other than the sections entitled "Election of Directors—Report of the Compensation Committee of the Board of Directors," "—Performance Graph," "—Report of the Audit Committee of the Board of Directors" and "Ratification of Selection of Independent Auditors—Principal Auditors Fees and Services").
•: The description of the common shares contained in our registration statement on Form 8-A filed on August 23, 2002 pursuant to Section 12 of the Exchange Act, including any amendment or report filed for the purpose of updating such description.

Arch Capital is also incorporating by reference the information contained in all other documents that it files with the SEC between the date of the initial filing of the registration statement of which this prospectus is a part and the effectiveness of the registration statement, as well as between the date of this prospectus and the time that all of the securities registered under the registration statement are sold. The information contained in any of these documents will be considered part of this prospectus from the date these documents are filed.

If you would like to receive a copy of any document incorporated by reference into this prospectus (which will not include any of the exhibits to the document other than those exhibits that are themselves specifically incorporated by reference into this prospectus) or Arch Capital's and Arch US's constitutional documents, you should call or write to Arch Capital Group Ltd., Wessex House, 45 Reid

Street, Hamilton HM 12 Bermuda, Attention: Secretary (telephone (441) 278-9250). Arch Capital will provide these documents, without charge, by first class mail.

None of Arch Capital or Arch US have authorized anyone to give any information or make any representation about Arch Capital or Arch US that is different from, or in addition to, that contained in this prospectus or in any of the materials that Arch Capital or Arch US have incorporated by reference into this prospectus. Therefore, if anyone does give you information of this sort, you should not rely on it. If you are in a jurisdiction where offers to exchange or sell, or solicitations of offers to exchange or purchase, the securities offered by this document or the solicitation of proxies is unlawful, or if you are a person to whom it is unlawful to direct these types of activities, then the offer presented in this prospectus does not extend to you. The information contained in this prospectus speaks only as of the date of this document, unless the information specifically indicates that another date applies.

ABOUT THIS PROSPECTUS

This prospectus is part of a registration statement that Arch Capital Group Ltd. and Arch Capital Group (U.S.) Inc. have filed with the Securities and Exchange Commission utilizing a "shelf" registration process. Under this shelf process, Arch Capital Group Ltd. and Arch Capital Group (U.S.) Inc. may sell the securities described in this prospectus in one or more offerings for up to an aggregate initial offering price of $650,000,000. This prospectus provides you with a general description of the securities that Arch Capital Group Ltd. and Arch Capital Group (U.S.) Inc. may offer. This prospectus does not contain all of the information set forth in the registration statement as permitted by the rules and regulations of the SEC. For additional information regarding Arch Capital Group Ltd. or Arch Capital Group (U.S.) Inc. and the offered securities, please refer to the registration statement. Each time Arch Capital Group Ltd. or Arch Capital Group (U.S.) Inc. sell securities, we will file a prospectus supplement with the SEC that will contain specific information about the terms of that offering. You should read both this prospectus and any prospectus supplement together with additional information described under the heading "Where You Can Find More Information."

In this prospectus and in the accompanying prospectus supplement, except as specified otherwise or unless the context requires otherwise: (a) "ACGL" and "Arch Capital" refer to Arch Capital Group Ltd. and not any of its subsidiaries, (b) "we," "us" and "our" refer to ACGL and its subsidiaries, (c) "Arch US" refers to Arch Capital Group (U.S.) Inc. (d) "Arch Re Bermuda" refers only to our wholly owned Bermuda reinsurance subsidiary, Arch Reinsurance Ltd., (e) "Arch Re U.S." refers only to our wholly owned U.S. reinsurance subsidiary, Arch Reinsurance Company, and (f) (i) "Arch Specialty" refers to Arch Specialty Insurance Company, (ii) "Arch E&S" refers to Arch Excess & Surplus Insurance Company, (iii) "Western Diversified" refers to Western Diversified Casualty Insurance Company, (iv) "PSIC" refers to Personal Service Insurance Company and (v) "American Independent" refers to American Independent Insurance Company, which along with Arch Insurance Company ("Arch Insurance") are our wholly owned U.S. insurance subsidiaries. The terms "Warburg Pincus funds" and "Hellman & Friedman funds" refer to investment funds affiliated with Warburg Pincus LLC and investment funds led by Hellman & Friedman LLC, respectively.

CAUTIONARY NOTE REGARDING FORWARD-LOOKING STATEMENTS

The Private Securities Litigation Reform Act of 1995 provides a "safe harbor" for forward-looking statements. This prospectus or any other written or oral statements made by or on behalf of us may include forward-looking statements which reflect our current views with respect to future events and financial performance. All statements other than statements of historical fact included in or incorporated by reference in this prospectus are forward-looking statements. Forward-looking statements can generally be identified by the use of forward-looking terminology such as "may", "will", "expect", "intend", "estimate", "anticipate", "believe" or "continue" or their negative or variations or similar terminology.

Forward-looking statements involve our current assessment of risks and uncertainties. Actual events and results may differ materially from those expressed or implied in these statements. Important factors that could cause actual events or results to differ materially from those indicated in such statements are discussed below and elsewhere in this prospectus, including, without limitation, the section entitled "Risk Factors," and in our periodic reports filed with the Securities and Exchange Commission and include:

•: our ability to successfully implement our business strategy during "soft" as well as "hard" markets;
•: acceptance of our business strategy, security and financial condition by rating agencies and regulators, as well as by brokers and our insureds and reinsureds;
•: our ability to maintain or improve our ratings, which may be affected by our ability to raise additional equity or debt financings, as well as other factors described herein;
•: general economic and market conditions (including inflation, interest rates and foreign currency exchange rates) and conditions specific to the reinsurance and insurance markets in which we operate;
•: competition, including increased competition, on the basis of pricing, capacity, coverage terms or other factors;
•: our ability to successfully integrate new management and operating personnel and to establish and maintain operating procedures (including the implementation of improved computerized systems and programs to replace and support manual systems) to effectively support our underwriting initiatives and to develop accurate actuarial data, especially in the light of the rapid growth of our business;
•: the loss of key personnel;
•: the integration of businesses we have acquired or may acquire into our existing operations;
•: accuracy of those estimates and judgments utilized in the preparation of our financial statements, including those related to revenue recognition, insurance and other reserves, reinsurance recoverables, investment valuations, intangible assets, bad debts, income taxes, contingencies and litigation, for a relatively new insurance and reinsurance company, like our company, are even more difficult to make than those made in a mature company since limited historical information has been reported to us through March 31, 2004;
•: greater than expected loss ratios on business written by us and adverse development on claim and/or claim expense liabilities related to business written by our insurance and reinsurance subsidiaries;
•: severity and/or frequency of losses;
•: claims for natural or man-made catastrophic events in our insurance or reinsurance business could cause large losses and substantial volatility in our results of operations;
•: acts of terrorism, political unrest and other hostilities or other unforecasted and unpredictable events;
•: losses relating to aviation business and business produced by a certain managing underwriting agency for which we may be liable to the purchaser of our prior reinsurance business or to others in connection with the May 5, 2000 asset sale described in our periodic reports filed with the SEC;

•: availability to us of reinsurance to manage our gross and net exposures and the cost of such reinsurance;
•: the failure of reinsurers, managing general agents or others to meet their obligations to us;
•: the timing of loss payments being faster or the receipt of reinsurance recoverables being slower than anticipated by us;
•: changes in accounting principles or the application of such principles by accounting firms or regulators;
•: statutory or regulatory developments, including as to tax policy and matters and insurance and other regulatory matters (such as the adoption of proposed legislation that would affect Bermuda-headquartered companies and/or Bermuda-based insurers or reinsurers); and
•: rating agency policies and practices.

LEGAL MATTERS

Certain legal matters with respect to the securities will be passed upon for us and Arch US by Cahill Gordon & Reindel LLP, New York, New York. Certain legal matters with respect to the securities under the laws of Bermuda will be passed upon for us by Conyers, Dill & Pearman, Hamilton, Bermuda.

EXPERTS

The financial statements incorporated in this Prospectus by reference to the Annual Report on Form 10-K of Arch Capital Group Ltd. for the year ended December 31, 2003 have been so incorporated in reliance on the report of PricewaterhouseCoopers LLP, independent registered public accounting firm, given on the authority of said firm as experts in auditing and accounting.

ENFORCEMENT OF CIVIL LIABILITIES UNDER UNITED STATES FEDERAL SECURITIES LAWS

We are a Bermuda company. In addition, certain of our directors and officers as well as certain of the experts named in this prospectus, reside outside the United States, and all or a substantial portion of our assets and their assets are located outside the United States. Therefore, it may be difficult for investors to effect service of process within the United States upon those persons or to recover against us or those persons on judgments of courts in the United States, including judgments based on civil liabilities provisions of the United States federal securities laws. However, investors may serve Arch Capital with process in the United States with respect to actions against us arising out of or in connection with violations of United States federal securities law relating to offers and sales of securities covered by this prospectus by serving National Registered Agents, Inc., 440 9th Avenue, 5th floor, New York, New York 10001, its United States agent irrevocably appointed for that purpose.

We have been advised by Conyers Dill & Pearman, our Bermuda counsel, that the United States and Bermuda do not currently have a treaty providing for reciprocal recognition and enforcement of judgments in civil and commercial matters. We also have been advised by Conyers Dill & Pearman that there is doubt as to whether the courts of Bermuda would enforce (1) judgments of United States courts based on the civil liability provisions of the United States federal securities laws obtained in actions against us or our directors and officers, and (2) original actions brought in Bermuda against us or our officers and directors based solely upon the United States federal securities laws. A Bermuda court may, however, impose civil liability on us or our directors or officers in a suit brought in the Supreme Court of Bermuda; provided that the facts alleged constitute or give rise to a cause of action under Bermuda law. Certain remedies available under the laws of U.S. jurisdictions, including certain remedies under the U.S. federal securities laws, would not be allowed in Bermuda courts to the extent that they are contrary to public policy.

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