Barring any unforeseen surprises, Fox will win both the May sweeps ratings period and the 2005-06 season in the advertiser-friendly adults 18 to 49 demographic.

Driven by its midseason phenomenon “American Idol,” Fox is also expected to be the highest-rated network in adults 18 to 34 and teens for May sweeps and the season.

The May sweeps began Thursday, April 27, and ends Wednesday, May 24, the same day the 2005-06 ratings season officially ends.

Fox won’t be the top network in every demographic for the season, however. CBS is expected to best Fox in total viewers and among adults 25 to 54.

As in the last May sweeps “Idol” is once again expected to be the highest-rated series for the sweeps and the season in adults 18 to 49, despite being in its fifth season.

“Logic purely dictates that as a show enters its third, fourth, fifth and sixth season one would naturally believe the show would erode,” Peter Liguori, Fox president of entertainment, said. “It strikes a chord with people. There’s football season; there’s baseball season; there’s ‘American Idol’ season.”

Fox’s success did not come solely from “Idol,” Mr. Liguori said, pointing to the success of shows such as “Prison break,” “24,” “Bones” and “House.” For 2005-06, Fox saw ratings growth in adults 18 to 49 on five out of seven nights a week in its entertainment programming (postseason baseball is excluded from the averages). On the other two nights, Monday and Friday, Fox is flat season to season.

“We did it on the back of entertainment across all nights and across all genres,” he said.