This sudden exit of so many marquee stations has not only renewed the perennial debate about the relative health of rock as a musical genre, but it also indicates that the alternative format, once the darling of radio a decade ago, is now taking perhaps the heaviest fire in the radio industry's battle to retain listeners in the face of Internet and satellite radio competition. Many rock stations may be in for another blow when the shock jock Howard Stern departs for Sirius Satellite Radio next year.

There are still signs that a fervent alternative scene survives. This weekend, for instance, 50,000 people a day are expected to visit Indio, Calif., for the sixth-annual Coachella Valley Music Festival, the biggest rock event of its kind in the United States, to cheer bands like the Arcade Fire and the Secret Machines. Moreover, while alternative programmers are searching for a solution, for the moment they have the benefit of new music by a clutch of reliable stars from the genre's heyday: Nine Inch Nails, Weezer and Beck are releasing their first albums in two years or more, and songs by each rocketed to the top of Billboard magazine's modern-rock airplay chart.

But many musicians in the newer bands on the alternative playlists "could be your waiter tomorrow night and you wouldn't know the difference," griped a radio promotion executive at one major label, who requested anonymity for fear of offending bands on his label.

Ratings for rock radio stations have been languishing for years. The share of the 18-to-34 age group that is tuning in to alternative stations has shrunk by more than 20 percent in the last five years, according to Arbitron, while stations playing rap and R&B or Spanish-language formats have enjoyed an expanding audience.

As a result, many rock programmers aren't sure what to play.

"The format in the last couple of years has gone through an identity crisis," said Kevin Weatherly, program director of KROQ, a closely watched alternative powerhouse in Los Angeles. "You have stations that are too cool, that move too quickly and are only playing the coolest music, which doesn't at the end of the day attract enough of the audience. Or you have the other extreme, dumb rock, red-state rock that the cool kids just flat out aren't into."