Valve announced that it has acquired Turtle Rock Studios, the Orange County-based development studio behind portions of Counter-Strike: Condition Zero, various Counter-Strike maps and the upcoming Left 4 Dead game.

The studio’s Left 4 Dead joins Valve’s game properties which includes the blockbuster Half-Life franchise, Counter-Strike, Team Fortress and Day of Defeat series of games, as well as Portal.

“We have been seeing very strong growth with Steam and Source, our content distribution and development platforms, up over 150% over the last 12 months. Given our expectations for Left 4 Dead and our long-standing relationships with members of the Turtle Rock team, this was an easy decision. It also gives us a base from which to expand our development activities in the Los Angeles area,” said Gabe Newell, president of Valve.

Left 4 Dead is now expected to be released this Summer on PC.

Left 4 Dead, the upcoming survival co-op game from Turtle Rock Studios and Valve, will be shown at Quakecon 2007 in Dallas, Texas August 2-5. It will be playable by attendees and members of the design team will be on hand to answer questions from visitors.

As typical of Valve Software, a very short press release from the makers of the Half-Life series announced a partnership with Turtle Rock Studios to create Left 4 Dead, a co-operative multiplayer action game.

“By leveraging its industry-leading AI technology and years of Counter-Strike development experience, Turtle Rock is building upon the elements that make Counter-Strike successful and applying them to Left 4 Dead,” said Gabe Newell, co-founder and president of Valve. “What Counter-Strike did for multiplayer action games, Left 4 Dead will do for co-op games.”

Left 4 Dead is being developed using the “most advanced version” of the Source engine and introduces version 2 of Turtle Rock’s AI technology. The game is set for release on PC sometime next year.