Conan O'Brien made his debut as the host of NBC's The Tonight Show on June 1.Conan O'Brien made his debut as the host of NBC's The Tonight Show on June 1. (Paul Drinkwater/NBC/Associated Press)

Conan O'Brien was king of late night television in his first week at the helm of NBC's The Tonight Show, but many of his opening-night viewers deserted him as the days went on, data from Nielsen Media Research shows.

O'Brien averaged 6.1 million viewers a night in the first week after his show opened on June 1, compared with his competitor, David Letterman, whose Late Show on CBS averaged 3.5 million viewers in the same period.

O'Brien got off to a strong start with 9.2 million viewers tuning in on his heavily promoted opening night show. But they dwindled steadily each subsequent night to 4.5 million viewers by June 5.

Jay Leno, O'Brien's immediate predecessor, had an average of seven million viewers in his last week as the show's host.

Preliminary Nielsen metered market ratings for the past week indicate that Letterman's Tuesday, June 9, show attracted more viewers than The Tonight Show for the first time since October, garnering a 3.4 rating in households compared with a 2.9 household rating for O'Brien.

On Wednesday, O'Brien was back in the top spot, with The Tonight Show scoring a 3.2 household rating and the Late Show coming in with a 3.1.

The final numbers won't be out until later in the week, but there's a strong chance that Letterman could average more viewers drawn by his dust-up with Sarah Palin. The Alaska governor called Letterman "perverted" after his crack about her daughter getting "knocked up" by New York Yankee Alex Rodriguez.

A back-and-forth race between O'Brien and Letterman seems likely in coming months as viewers develop their loyalties.

O'Brien spent 17 years as host of NBC's Late Night and his move to an earlier time slot has been in the works for five years.

Leno will host a weeknight prime-time show on NBC, starting in September.

Letterman has agreed to stay on as host of the Late Show through the 2011-12 television season.

With files from The Associated Press