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: Stanfill John L (Sr. VP-Sales, Customer Service)

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	8000	$70.57	Disposed	80693	Direct
2012-05-07	Common Stock, par value $0.01 per share	M	750	$19.71	Acquired	81443	Direct
2012-05-07	Common Stock, par value $0.01 per share	M	2602	$25.00	Acquired	84045	Direct
2012-05-07	Common Stock, par value $0.01 per share	M	4600	$42.29	Acquired	88645	Direct
2012-05-07	Common Stock, par value $0.01 per share	M	3900	$57.16	Acquired	92545	Direct
2012-05-07	Common Stock, par value $0.01 per share	S	11852	$70.13	Disposed	80693	Direct

Derivative Transactions

Date	Security	Exercise Price	Code	Shares	A/D	Expiration	Underlying	Ownership
2012-05-07	Option to Acquire Common Stock	$19.71	M	750	Disposed	2012-08-05	Common Stock (750)	Direct
2012-05-07	Option to Acquire Common Stock	$25.00	M	2602	Disposed	2019-03-01	Common Stock (2602)	Direct
2012-05-07	Option to Acquire Common Stock	$42.29	M	4600	Disposed	2020-03-11	Common Stock (4600)	Direct
2012-05-07	Option to Acquire Common Stock	$57.16	M	3900	Disposed	2021-03-03	Common Stock (3900)	Direct

Footnotes

F1: Average based on sales prices ranging from $70.40 to $70.65.

F2: Represents all shares of Common Stock owned, consisting of 2,592 shares of Common Stock and 78,101 shares of Common Stock subject to unvested Restricted Stock Grants.

F3: Represents all shares of Common Stock owned, consisting of 3,342 shares of Common Stock and 78,101 shares of Common Stock subject to unvested Restricted Stock Grants.

F4: Represents all shares of Common Stock owned, consisting of 5,944 shares of Common Stock and 78,101 shares of Common Stock subject to unvested Restricted Stock Grants.

F5: Represents all shares of Common Stock owned, consisting of 10,544 shares of Common Stock and 78,101 shares of Common Stock subject to unvested Restricted Stock Grants.

F6: Represents all shares of Common Stock owned, consisting of 14,444 shares of Common Stock and 78,101 shares of Common Stock subject to unvested Restricted Stock Grants.

F7: Average based on sales prices ranging from $69.94 to $70.39.

F8: Represents all shares of Common Stock owned, consisting of 2,592 shares of Common Stock and 78,101 shares of Common Stock subject to unvested Restricted Stock Grants.

F9: The option vested in four equal installments on August 6, 2003, August 6, 2004, August 6, 2005 and August 6, 2006.

F10: The option vested in three equal installments on March 2, 2010, March 2, 2011 and March 2, 2012.

F11: The option vests in three equal installments on March 12, 2011, March 2, 2012 and March 2, 2013.

F12: The option vests in three equal installments on March 4, 2012, March 4, 2013 and March 4, 2014.