Now the largest U.S. carrier is testing the limits of passengers' willingness to pay for extras, announcing plans to start charging $15 for what many fliers consider a downright necessity - a single checked bag.

American Airlines said Wednesday that beginning with travel June 15 it would charge passengers if they are flying on a discounted coach fare. That comes just two weeks after most airlines began charging for checking a second bag.

Many customers are asking what's next?

''Pay toilets in the coach cabin, maybe,'' joked longtime airline consultant Mike Boyd.

The new charge is part of a larger effort by the industry to find ways to improve revenue in the face of soaring fuel prices. The airlines anticipated a multibillion-dollar loss this year, even before oil's latest spike above $130 a barrel Wednesday.

Still, the latest effort could be a tough sell with passengers.

''It's ridiculous,'' said Dave Boydston, an Ogden passenger waiting to collect his bags Wednesday at Salt Lake City International Airport. "Obviously they've got to do something to offset the rising fuel cost, but you've got to have clothes wherever you are going. So you've got to have a bag."

American announced the change at the same time it said it would slash capacity and retire at least 75 older, gas-guzzling planes. Gerard Arpey, CEO of American parent AMR Corp., said the industry must find ways to cover rising costs.

If so, American and other airlines that might follow its lead are in for some push-back from plenty of customers.

"Are you kidding?" said Lu Unger, who was waiting to board a United Airlines flight from Salt Lake to Denver. "I don't like that. I'm not going to fly them if I have to pay $15. I'll fly any other airline."

A spokeswoman for Delta Air Lines, the dominant carrier at the Salt Lake airport, said the airline has no plans to match American's fee, but she cautioned that Delta is considering all options with oil at $130 per barrel.

American is the first major carrier to charge for a single bag, but it is not the only U.S. airline to do so. Last June, low-cost carrier Spirit Airlines said it would begin charging $5 for the first bag when passengers booked online, or $10 otherwise. Spirit's online charge is set to rise to $10 next month.

With American leading the way, other airlines will be tempted to impose baggage charges, too.

''We are actually seriously studying it,'' said Robin Urbanski, a spokeswoman for United Airlines. No other carriers announced plans to match the fee, but like United, not all ruled it out.

The extra baggage fees will only exacerbate the situation on already-crowded planes, said Patricia Friend, the president of the Association of Flight Attendants, a union representing 55,000 employees at 20 airlines.

''Too many passengers will now try to carry on even more rather than pay a fee,'' Friend said.

Some passengers were resigned that flying will continue to get more expensive.

''It's still cheaper than gas,'' said passenger Rick Mattingly at Chicago's O'Hare Airport. ''If you look at it that way, 15 bucks is pretty cheap."