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 *
FLORANCE ANDREW C
   2. Issuer Name and Ticker or Trading Symbol
COSTAR GROUP INC [CSGP] 		5. Relationship of Reporting Person(s) to Issuer
(Check all applicable)
__X__ Director _____ 10% Owner
__X__ Officer (give title below) _____ Other (specify below)
President and CEO
(Last)
(First)
(Middle)
C/O COSTAR GROUP, INC., 2 BETHESDA METRO CENTER, 10TH FLOOR		 3. Date of Earliest Transaction (Month/Day/Year)
10/27/2009
(Street)

BETHESDA, MD 20814-5388		 4. If Amendment, Date Original Filed(Month/Day/Year)
 6. Individual or Joint/Group Filing(Check Applicable Line)
_X_ Form filed by One Reporting Person
___ 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 Stock, par value $0.01 per share 10/27/2009   M   12,940 A $ 24.875 232,861 (1) D  
Common Stock, par value $0.01 per share 10/27/2009   S   12,940 D $ 41.03 (2) 219,921 (3) D  
Common Stock, par value $0.01 per share 10/27/2009   M   22,000 A $ 18.06 241,921 (4) D  
Common Stock, par value $0.01 per share 10/27/2009   S   22,000 D $ 40.56 (5) 219,921 (6) D  
Common Stock, par value $0.01 per share 10/28/2009   S   8,000 D $ 39.76 (7) 211,921 (8) D  
Common Stock, par value $0.01 per share 10/29/2009   S   3,600 D $ 39.34 (9) 208,321 (10) D  

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
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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
Option to Acquire Common Stock $ 24.875 10/27/2009   M     12,940   (11) 06/20/2010 Common Stock 12,940 $ 0 0 D  
Option to Acquire Common Stock $ 18.06 10/27/2009   M     22,000   (12) 04/16/2011 Common Stock 22,000 $ 0 22,463 D  

Reporting Owners

Reporting Owner Name / Address Relationships
 Director  10% Owner  Officer  Other
FLORANCE ANDREW C
C/O COSTAR GROUP, INC.
2 BETHESDA METRO CENTER, 10TH FLOOR
BETHESDA, MD 20814-5388		   X     President and CEO  

Signatures

 /s/ Jonathan Coleman, Attorney-in-Fact   10/29/2009
**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) Represents all shares of Common Stock owned, consisting of 148,936 shares of Common Stock and 83,925 shares of Common Stock subject to unvested Restricted Stock Grants.
(2) Average based on sale prices ranging from $40.90 to $41.30.
(3) Represents all shares of Common Stock owned, consisting of 135,996 shares of Common Stock and 83,925 shares of Common Stock subject to unvested Restricted Stock Grants.
(4) Represents all shares of Common Stock owned, consisting of 157,996 shares of Common Stock and 83,925 shares of Common Stock subject to unvested Restricted Stock Grants.
(5) Average based on sale prices ranging from $40.49 to $40.70.
(6) Represents all shares of Common Stock owned, consisting of 135,996 shares of Common Stock and 83,925 shares of Common Stock subject to unvested Restricted Stock Grants.
(7) Average based on sale prices ranging from $39.53 to $40.12.
(8) Represents all shares of Common Stock owned, consisting of 127,996 shares of Common Stock and 83,925 shares of Common Stock subject to unvested Restricted Stock Grants.
(9) Average based on sale prices ranging from $39.30 to $39.38.
(10) Represents all shares of Common Stock owned, consisting of 124,396 shares of Common Stock and 83,925 shares of Common Stock subject to unvested Restricted Stock Grants.
(11) The option vested in four equal installments on June 21, 2001, June 21, 2002, June 21, 2003 and June 21, 2004.
(12) The option vested in four equal installments on April 17, 2002, April 17, 2003, April 17, 2004 and April 17, 2005.

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