COSTAR GROUP, INC. — Director's Dealing 2012

May 8, 2012

SEC Form 4 — Statement of Changes in Beneficial Ownership

Issuer: COSTAR GROUP INC (CSGP)
CIK: 0001057352
Period of Report: 2012-05-04

Reporting Person: Radecki Brian J (Chief Financial Officer)

Non-Derivative Transactions

Date	Security	Code	Shares	Price	A/D	Holdings After	Ownership
2012-05-04	Common Stock, par value $0.01 per share	S	2711	$70.78	Disposed	71461	Direct
2012-05-07	Common Stock, par value $0.01 per share	M	1875	$18.28	Acquired	73336	Direct
2012-05-07	Common Stock, par value $0.01 per share	M	5940	$39.81	Acquired	79276	Direct
2012-05-07	Common Stock, par value $0.01 per share	S	7815	$70.50	Disposed	71461	Direct

Derivative Transactions

Date	Security	Exercise Price	Code	Shares	A/D	Expiration	Underlying	Ownership
2012-05-07	Option to Acquire Common Stock	$18.28	M	1875	Disposed	2012-12-01	Common Stock (1875)	Direct
2012-05-07	Option to Acquire Common Stock	$39.81	M	5940	Disposed	2014-02-05	Common Stock (5940)	Direct

Footnotes

F1: Average based on sales prices ranging from $70.69 to $70.82.

F2: Represents all shares of Common Stock owned, consisting of 20,993 shares of Common Stock and 50,468 shares of Common Stock subject to unvested Restricted Stock Grants.

F3: Represents all shares of Common Stock owned, consisting of 22,868 shares of Common Stock and 50,468 shares of Common Stock subject to unvested Restricted Stock Grants.

F4: Represents all shares of Common Stock owned, consisting of 28,808 shares of Common Stock and 50,468 shares of Common Stock subject to unvested Restricted Stock Grants.

F5: Represents all shares of Common Stock owned, consisting of 20,993 shares of Common Stock and 50,468 shares of Common Stock subject to unvested Restricted Stock Grants.

F6: The option vested in four equal installments on December 2, 2003, December 2, 2004, December 2, 2005 and December 2, 2006.

F7: The option vested in four equal installments on February 6, 2005, February 6, 2006, February 6, 2007 and February 6, 2008.