COSTAR GROUP, INC. — Director's Dealing 2012

May 15, 2012

SEC Form 4 — Statement of Changes in Beneficial Ownership

Issuer: COSTAR GROUP INC (CSGP)
CIK: 0001057352
Period of Report: 2012-05-11

Reporting Person: Radecki Brian J (Chief Financial Officer)

Non-Derivative Transactions

Date	Security	Code	Shares	Price	A/D	Holdings After	Ownership
2012-05-11	Common Stock, par value $0.01 per share	M	60	$39.81	Acquired	71521	Direct
2012-05-11	Common Stock, par value $0.01 per share	S	60	$70.50	Disposed	71461	Direct

Derivative Transactions

Date	Security	Exercise Price	Code	Shares	A/D	Expiration	Underlying	Ownership
2012-05-11	Option to Acquire Common Stock	$39.81	M	60	Disposed	2014-02-05	Common Stock (60)	Direct

Footnotes

F1: Represents all shares of Common Stock owned, consisting of 21,053 shares of Common Stock and 50,468 shares of Common Stock subject to unvested Restricted Stock Grants.

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