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 *
Stanfill John L
  2. Issuer Name and Ticker or Trading Symbol
COSTAR GROUP INC [CSGP]
5. Relationship of Reporting Person(s) to Issuer
(Check all applicable)
_____ Director _____ 10% Owner
__X__ Officer (give title below) _____ Other (specify below)
Sr. VP-Sales, Customer Service
(Last)
(First)
(Middle)
C/O COSTAR GROUP, INC., 1331 L STREET, NW
3. Date of Earliest Transaction (Month/Day/Year)
03/11/2013
(Street)

WASHINGTON, DC 20005
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 03/11/2013   A   6,000 A $ 0 50,184 (1) D  
Common Stock, par value $0.01 per share 03/12/2013   M   2,300 A $ 42.29 52,484 (2) D  
Common Stock, par value $0.01 per share 03/12/2013   M   3,900 A $ 57.16 56,384 (3) D  
Common Stock, par value $0.01 per share 03/12/2013   M   196 A $ 39.53 56,580 (4) D  
Common Stock, par value $0.01 per share 03/12/2013   M   3,866 A $ 58.95 60,446 (5) D  
Common Stock, par value $0.01 per share 03/12/2013   S   14,599 D $ 101.1 (6) 45,847 (7) D  
Common Stock, par value $0.01 per share 03/12/2013   F   486 D $ 101.06 45,361 (8) 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 $ 102.16 03/11/2013   A   10,100     (9) 03/10/2023 Common Stock 10,100 $ 0 10,100 D  
Option to Acquire Common Stock $ 42.29 03/12/2013   M     2,300   (10) 03/11/2020 Common Stock 2,300 $ 0 0 D  
Option to Acquire Common Stock $ 57.16 03/12/2013   M     3,900   (11) 03/03/2021 Common Stock 3,900 $ 0 3,900 D  
Option to Acquire Common Stock $ 39.53 03/12/2013   M     196   (12) 05/04/2014 Common Stock 196 $ 0 0 D  
Option to Acquire Common Stock $ 58.95 03/12/2013   M     3,866   (13) 02/20/2022 Common Stock 3,866 $ 0 7,734 D  

Reporting Owners

Reporting Owner Name / Address Relationships
 Director  10% Owner  Officer  Other
Stanfill John L
C/O COSTAR GROUP, INC.
1331 L STREET, NW
WASHINGTON, DC 20005
      Sr. VP-Sales, Customer Service  

Signatures

 /s/ Jonathan Coleman, Attorney-in-Fact   03/13/2013
**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 5,449 shares of Common Stock and 44,735 shares of Common Stock subject to unvested Restricted Stock Grants.
(2) Represents all shares of Common Stock owned, consisting of 7,749 shares of Common Stock and 44,735 shares of Common Stock subject to unvested Restricted Stock Grants.
(3) Represents all shares of Common Stock owned, consisting of 11,649 shares of Common Stock and 44,735 shares of Common Stock subject to unvested Restricted Stock Grants.
(4) Represents all shares of Common Stock owned, consisting of 11,845 shares of Common Stock and 44,735 shares of Common Stock subject to unvested Restricted Stock Grants.
(5) Represents all shares of Common Stock owned, consisting of 15,711 shares of Common Stock and 44,735 shares of Common Stock subject to unvested Restricted Stock Grants.
(6) Average based on sales prices ranging from $100.75 to $102.35.
(7) Represents all shares of Common Stock owned, consisting of 1,112 shares of Common Stock and 44,735 shares of Common Stock subject to unvested Restricted Stock Grants.
(8) Represents all shares of Common Stock owned, consisting of 1,660 shares of Common Stock and 43,701 shares of Common Stock subject to unvested Restricted Stock Grants.
(9) The option vests in three equal installments on March 11, 2014, March 11, 2015, and March 11, 2016.
(10) The option vested in three equal installments on March 12, 2011, March 12, 2012, and March 12, 2013.
(11) The option vests in three equal installments on March 4, 2012, March 4, 2013, and March 4, 2014.
(12) The option vested in three equal installments on May 5, 2005, May 5, 2006, May 5, 2007, and May 5, 2008.
(13) The option vests in three equal installments on February 21, 2013, February 21, 2014, and February 21, 2015.

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