COSTAR GROUP, INC. — Director's Dealing 2010

Aug 6, 2010

SEC Form 4 — Statement of Changes in Beneficial Ownership

Issuer: COSTAR GROUP INC (CSGP)
CIK: 0001057352
Period of Report: 2010-08-04

Reporting Person: FLORANCE ANDREW C (Director, President and CEO)

Non-Derivative Transactions

Date	Security	Code	Shares	Price	A/D	Holdings After	Ownership
2010-08-04	Common Stock, par value $0.01 per share	M	8725	$18.06	Acquired	231202	Direct
2010-08-04	Common Stock, par value $0.01 per share	S	8725	$44.35	Disposed	222477	Direct
2010-08-05	Common Stock, par value $0.01 per share	M	3855	$18.06	Acquired	226332	Direct
2010-08-05	Common Stock, par value $0.01 per share	S	3855	$44.08	Disposed	222477	Direct

Derivative Transactions

Date	Security	Exercise Price	Code	Shares	A/D	Expiration	Underlying	Ownership
2010-08-04	Option to Acquire Common Stock	$18.06	M	8725	Disposed	2011-04-16	Common Stock (8725)	Direct
2010-08-05	Option to Acquire Common Stock	$18.06	M	3855	Disposed	2011-04-16	Common Stock (3855)	Direct

Footnotes

F1: Represents all shares of Common Stock owned, consisting of 144,951 shares of Common Stock and 86,251 shares of Common Stock subject to unvested Restricted Stock Grants.

F2: Average based on sales prices ranging from $44.20 to $44.54.

F3: Represents all shares of Common Stock owned, consisting of 136,226 shares of Common Stock and 86,251 shares of Common Stock subject to unvested Restricted Stock Grants.

F4: Represents all shares of Common Stock owned, consisting of 140,081 shares of Common Stock and 86,251 shares of Common Stock subject to unvested Restricted Stock Grants.

F5: Average based on sales prices ranging from $44.00 to $44.27.

F6: The option vested in four equal installments on April 17, 2002, April 17, 2003, April 17, 2004 and April 17, 2005.