May 22 (Bloomberg) -- News Corp.'s Fox, the U.S. television network with the top-rated ``American Idol,'' said it finished the season with the largest prime-time audience for the first time, surpassing five-year champion CBS.

The network also drew the most viewers ages 18 to 49, the group advertisers pay the most to reach, for a fourth straight year, according to preliminary data from Nielsen Media Research. Last night Fox had 31.7 million viewers, up 2.9 percent from a year ago, for the two-hour finale of ``Idol'' as 25-year-old rocker David Cook was crowned the seventh-season winner.

Fox was the only major network to increase its audience this year as the 100-day Hollywood writers strike sent most scripted shows into reruns. The network averaged 11.2 million viewers a night for the season that runs from September to May, up 7.7 percent from a year earlier, Fox said in a statement.

``What catapulted them is probably that they were the least harmed from the writers strike,'' said Brad Adgate, research director at New York-based ad agency Horizon Media Inc. ``They had a lot of unscripted shows that were not as impacted, like `American Idol.'''

Fox dominated the 18-to-49 category, with 2.4 percent more viewers. CBS Corp. saw its audience in that age group drop 19 percent and tied for second place with Walt Disney Co.'s ABC, down 14 percent, according to preliminary Nielsen data supplied by Fox. General Electric Co.'s NBC lost 9.7 percent of its 18- to-49 viewers to finish fourth and the CW retreated 15 percent.

Season Interrupted

The season, which began in September, was interrupted when film and TV members of the Writers Guild of America walked off the job Nov. 5. The strike halted production of scripted shows and limited competition against the ``Idol'' reality show and Fox's other unscripted programs such as ``America's Most Wanted'' and ``Cops.''

CBS, based in New York, was the first broadcaster to return with new scripted episodes after writers returned to work in February. NBC finished in last place among the top four networks for the fifth consecutive year.

News Corp., based in New York, rose 17 cents to $19.05 at 4:01 p.m. in New York Stock Exchange composite trading. The Class A shares have declined 7 percent this year. CBS gained 10 cents to $23 and Disney, based in Burbank, California, fell 5 cents to $33.61. GE rose 2 cents to $31.01.

``Idol'' finished as the most popular prime-time show for a fifth-straight season, helping Fox vault past CBS to the top spot among all viewers. Fox first passed CBS in total audience in March.

Rock Fans

Cook, from Blue Springs, Missouri, beat 17-year-old David Archuleta with 56 percent of the record 97.5 million votes cast, ``Idol'' announced last night during the live broadcast. The rocker won over fans with renditions of ``Billie Jean'' and Mariah Carey's ``Always Be My Baby.''

Cook decided to try out for the show on a whim when he accompanied his brother to the audition.

Last year 30.8 million people watched as Jordin Sparks became the youngest American Idol at 17 years old. About 74 million votes were cast by telephone and text message then.

Judges Randy Jackson, Paula Abdul and Simon Cowell had pinned Archuleta as an early favorite to take the championship after he lured fans with a smooth voice and his rendition of the song ``Imagine.''

Ad agency researcher Adgate said he had expected the finale's audience to shrink about 10 percent from last year. Two strong finalists and the perception of a close vote helped the show increase viewers last night despite declining ratings earlier in the season, he said.

`Idol' Changes

Next year, Fox plans to shorten the Wednesday night results shows to half an hour and is considering other changes after ratings slipped.

``Both the network and the producers really want to take a look at the show for next year and see what we can do to inject it with new levels of energy and new levels of unpredictable twists and turns,'' Peter Liguori, chairman of Fox entertainment, said last week when the network announced its 2008-2009 TV lineup.

Fox got at least $780,000 for advance 30-second spots on ``Idol'' before the strike, about 30 percent more than a year earlier, ad buyers with knowledge of the deals said in January.

To contact the reporter on this story: Sarah Rabil in New York at srabil@bloomberg.net

Last Updated: May 22, 2008 16:12 EDT