LOS ANGELES (Reuters) - Conan O'Brien ruled late-night television in his first week hosting "The Tonight Show," but viewers deserted him as the days progressed, complete ratings numbers released on Thursday showed.
O'Brien averaged 6.1 million viewers a night last week during his much-hyped opening stand on NBC's "The Tonight Show," compared to an average of 3.5 million viewers for his main competitor, CBS' "Late Show with David Letterman."
O'Brien's predecessor, Jay Leno, averaged 7.0 million viewers in his last week, ended May 29. O'Brien's debut on June 1 attracted 9.2 million viewers, the program's biggest Monday-night audience in four years. But by Friday, his number fell to 4.5 million viewers.
And preliminary data showed Letterman this past Tuesday beat "The Tonight Show" for the first time since October, attracting a 3.4 rating in households compared to a 2.9 rating for O'Brien.
O'Brien was back on top on Wednesday night, and ratings experts said they expect a back-and-forth race between O'Brien and Letterman in future months.
"It's not about one week, it's about how do you hold the audience season after season," said Larry Gerbrandt, a principal at Media Valuation Partners.
Letterman, 62, has agreed to stay on as host of "Late Show" through the 2011-12 television season, a source close to the situation said on Thursday.
Meanwhile, fourth-ranked NBC has lauded O'Brien's performance and touted the 46-year-old comedian's ability to bring in younger viewers. In his first week on "The Tonight Show," O'Brien increased the program's performance in the advertiser-friendly adults 18-49 demographic to a 2.3 rating, compared to an average 1.4 rating when Leno was hosting.
"I was very pleased with the fact that Conan looked at home, he was in charge," said Rick Ludwin, executive vice president, late night and primetime for NBC Entertainment.
"'The Tonight Show' continues to be the big tent show," he said. "This is the show the whole family and viewers of all ages are invited to watch."
The late-night picture is expected to get more uncertain for O'Brien in September, when Leno begins hosting a new comedy show in the 10 p.m. time slot on NBC. Leno hosted "The Tonight Show" for 17 years, and experts have suggested that some of O'Brien's current viewers could opt for Leno's show before calling it a night.
Editing by Dean Goodman