UPN and the WB will cease operations this fall to make way for a new broadcast network called the CW, aimed at young, ethnic viewers, CBS Corporation and Warner Bros. Entertainment announced yesterday.

By shutting down the two small networks that jousted over the same pool of 18- to 34-year-old viewers and joining forces to capture that audience, the companies are essentially bowing to the reality of an increasingly competitive marketplace, executives acknowledged.

Both companies will own 50% of the new venture, which will be carried by Tribune Company stations and CBS-owned affiliates that carry UPN. They will offer 30 hours of programming a week, drawing from a pool of shows such as “Smallville,” “Everybody Hates Chris,” and “Beauty and the Geek.” The new venture will likely mean significant job losses, most from Burbank, Calif.-based WB, where 240 now work. And there will be fewer outlets of programming for producers, writers, and television studios.

The news about the launch of the new network – which will dramatically reshape the broadcast television landscape – was kept tightly under wraps until yesterday morning, when reporters were summoned to a news conference at the St. Regis Hotel in Manhattan.

“We’re coming here with a pretty historic announcement,” the president and CEO of CBS, Leslie Moonves, said.

“The CW is going to be a real competitor, a destination for young audiences, diverse audiences, and a real favorite with advertisers,” Mr. Moonves added. “The CW will be able to do something truly remarkable: program already hit shows every single day of the week, programs that consistently rank no. 1 or no. 2 in their time slots in the most coveted young adult demographic.” The chairman and chief executive officer of Warner Bros. Entertainment, Barry Meyer, called the new network “a partnership whose time has come.”

“The CW is a brand new network that will be a 50-50 partnership between Warner Bros. Entertainment and the CBS Corporation, with both companies contributing the best of their strategic assets – assets including existing hit television series, access to new programming, the strongest executive talent from both the WB and UPN, and a solid national distribution system,” he said.

The new network, which will begin airing in September, takes its name from the first initials of each parent company. “We couldn’t call it the WC for obvious reasons,” Mr. Moonves quipped.

As the fifth broadcast network, it will compete against ABC, CBS, Fox, and NBC – along with a slew of cable channels.