Ownership Submission
FORM 4
Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).
UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP OF SECURITIES

Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940
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(Print or Type Responses)
1. Name and Address of Reporting Person *
WARBURG PINCUS LLC
   2. Issuer Name and Ticker or Trading Symbol
ARCH CAPITAL GROUP LTD. [ACGL] 		5. Relationship of Reporting Person(s) to Issuer
(Check all applicable)
_____ Director __X__ 10% Owner
_____ Officer (give title below) _____ Other (specify below)
(Last)
(First)
(Middle)
466 LEXINGTON AVENUE		 3. Date of Earliest Transaction (Month/Day/Year)
12/30/2005
(Street)

NEW YORK, NY 10017		 4. If Amendment, Date Original Filed(Month/Day/Year)
 6. Individual or Joint/Group Filing(Check Applicable Line)
___ Form filed by One Reporting Person
_X_ Form filed by More than One Reporting Person
(City)
(State)
(Zip)
Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1.Title of Security
(Instr. 3)		 2. Transaction Date (Month/Day/Year) 2A. Deemed Execution Date, if any (Month/Day/Year) 3. Transaction Code
(Instr. 8)		 4. Securities Acquired (A) or Disposed of (D)
(Instr. 3, 4 and 5)		 5. Amount of Securities Beneficially Owned Following Reported Transaction(s)
(Instr. 3 and 4)		 6. Ownership Form: Direct (D) or Indirect (I)
(Instr. 4)		 7. Nature of Indirect Beneficial Ownership
(Instr. 4)
Code V Amount (A) or (D) Price
Common Shares, $0.01 par value per share (1) 12/30/2005   C   22,906,560 A (2) 22,910,478 I Partnership (1)

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Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned
(e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security
(Instr. 3)		 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (Month/Day/Year) 3A. Deemed Execution Date, if any (Month/Day/Year) 4. Transaction Code
(Instr. 8)		 5. Number of Derivative Securities Acquired (A) or Disposed of (D)
(Instr. 3, 4, and 5)		 6. Date Exercisable and Expiration Date
(Month/Day/Year)		 7. Title and Amount of Underlying Securities
(Instr. 3 and 4)		 8. Price of Derivative Security
(Instr. 5)		 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s)
(Instr. 4)		 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I)
(Instr. 4)		 11. Nature of Indirect Beneficial Ownership
(Instr. 4)
Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares
Series A Convertible Preference Shares (2) 12/30/2005   C(2)     20,906,560   (2)   (2) Common Shares 20,906,560 (2) 0 I Partnership (1)

Reporting Owners

Reporting Owner Name / Address Relationships
 Director  10% Owner  Officer  Other
WARBURG PINCUS LLC
466 LEXINGTON AVENUE
NEW YORK, NY 10017		     X    
WARBURG PINCUS PRIVATE EQUITY VIII L P
466 LEXINGTON AVENUE
NEW YORK, NY 10017		     X    

Signatures

 /s/Scott A. Arenare, Managing Director   12/30/2005
**Signature of Reporting Person Date

Explanation of Responses:

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).
** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
(1) THE SECURITY HOLDERS ARE WARBURG PINCUS (BERMUDA) PRIVATE EQUITY VIII, L.P. ("WP VIII BERMUDA") AND WARBURG PINCUS (BERMUDA) INTERNATIONAL PARTNERS, L.P. ("WPIP BERMUDA") AND WARBURG PINCUS NETHERLANDS INTERNATIONAL PARTNERS I, C.V. ("WPIP NETHERLANDS I"). WARBURG PINCUS (BERMUDA) PRIVATE EQUITY LTD. ("WP VIII BERMUDA LTD.") IS THE SOLE GENERAL PARTNER OF WP VIII BERMUDA. WARBURG PINCUS (BERMUDA) INTERNATIONAL LTD. ("WPIP BERMUDA LTD.") IS THE SOLE GENERAL PARTNER OF WPIP BERMUDA. WARBURG, PINCUS PARTNERS, LLC IS THE SOLE GENERAL PARTNER OF WPIP NETHERLANDS I. WP VIII BERMUDA, WPIP BERMUDA AND WPIP NETHERLANDS I ARE MANAGED BY WARBURG PINCUS LLC ("WP LLC").
(2) ON DECEMBER 30, 2005, THE REPORTING PERSONS CONVERTED AN AGGREGATE OF 20,906,560 SERIES A CONVERTIBLE PREFERENCE SHARES OF THE ISSUER INTO AN EQUAL NUMBER OF COMMON SHARES OF THE ISSUER.

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