COSTAR GROUP, INC. — Director's Dealing 2012

May 24, 2012

SEC Form 4 — Statement of Changes in Beneficial Ownership

Issuer: COSTAR GROUP INC (CSGP)
CIK: 0001057352
Period of Report: 2012-05-22

Reporting Person: Radecki Brian J (Chief Financial Officer)

Non-Derivative Transactions

Date	Security	Code	Shares	Price	A/D	Holdings After	Ownership
2012-05-22	Common Stock, par value $0.01 per share	M	19500	$25.00	Acquired	90961	Direct
2012-05-22	Common Stock, par value $0.01 per share	S	19500	$71.00	Disposed	71461	Direct
2012-05-22	Common Stock, par value $0.01 per share	S	1253	$71.50	Disposed	70208	Direct
2012-05-23	Common Stock, par value $0.01 per share	S	4509	$71.75	Disposed	65699	Direct

Derivative Transactions

Date	Security	Exercise Price	Code	Shares	A/D	Expiration	Underlying	Ownership
2012-05-22	Option to Acquire Common Stock	$25.00	M	19500	Disposed	2019-03-01	Common Stock (19500)	Direct

Footnotes

F1: Represents all shares of Common Stock owned, consisting of 40,493 shares of Common Stock and 50,468 shares of Common Stock subject to unvested Restricted Stock Grants.

F2: Average based on sales prices ranging from $71.00 to $71.08.

F3: 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.

F4: Represents all shares of Common Stock owned, consisting of 19,740 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 15,231 shares of Common Stock and 50,468 shares of Common Stock subject to unvested Restricted Stock Grants.

F6: The option vested in three equal installments on March 2, 2010, March 2, 2011, and March 2, 2012.