Keith Moon, 31, antic drummer for the British rock group The Who, died in his sleep early Thursday in his apartment here.

He was found by his fiancee, Annette Water-Lax, and taken to Middlesex Hospital, where he was pronounced dead. Cause of death was not immediately determined. A hospital spokesman said the coroner was investigating.

The small, manic Mr. Moon joined up with Peter Townshend, Roger Daltrey and John Entwistle in 1964 to form a rock group. As the group's popularity increased in the later '60s, it developed a violent, explosive personality that contrasted with the boy-next-door image of the Beatles and the rebellious character of the Rolling Stones. The Who's concert trademark was the finale, during which musical equipment flew everywhere and Townshend's guitars were demolished. Director Michelangelo Antonioni wrote just such a demolition scene into his movie "Blow-Up," but used another British rock group, The Yard-birds, reportedly because The Who were too difficult to control.

Mr. Moon, considered one of rock's finest drummers as well as its most rambunctions, indulged to the hilt a lewd, leering persona. When Townshend's innovative rock opera "Tommy," about a blind, deaf-mute messiah, was turned into a movie in 1975, Mr. Moon portrayed the trench-coated deviate, Uncle Ernie.

In 1970, while trying to escape a pursuing crowd of fans outside a Hatfield, England discotheque, Mr. Moon struck and fatally injured a bystander with his limousine. No charges were filed against him.

In 1973, The Who caused $6,000 worth of damage to a Montreal hotel room, but paid the bill in cash before leaving.

Townshend, Daltrey, Entwistle and Mr. Moon stopped working together in 1975, but reunited in the studio earlier this year to record "Who Are You." Just released, the album entered the national charts this week in the No. 20 spot.

"We're all very pleased," Mr. Moon said at the time of recording. "We just learned how to be The Who again."

A spokesman for Music Corporation of America said Mr. Moon had attended a movie premiere and lavish party honoring the late rock 'n' roller Buddy Holly thrown by ex-Beatle Paul McCartney Wednesday night.