The airline, the nation's sixth-biggest and Philadelphia International Airport's largest, with 61 percent of passengers, said it needed to generate more revenue to pay for fuel. It said the policy would take effect for travel that began on or after May 5.

And what about a third bag? That would cost you at least $100.

The fee mirrors a change United Airlines made Feb. 4. Airline-industry experts say it is likely that most other major carriers will adopt the same extra-bag fee by summer.

US Airways Group Inc. said the $25 fee for a second bag would not apply to frequent fliers who traveled more than 25,000 miles a year, first- or business-class customers, active-duty military personnel in uniform, unaccompanied minors, or those checking assistive devices, such as a wheelchair. US Airways said the change would affect about 8 percent of its customers who now check more than one bag.

One US Airways frequent flier, who probably will not have to pay the $25 fee, said he nevertheless wondered what the airline's managers, based in Tempe, Ariz., were thinking, given the problems they have had ensuring good baggage service at the Philadelphia hub.

"Now they're going to charge you to lose the second bag?" asked a dismayed Rich Raiders, an environmental engineer who lives in Drexel Hill and flies practically every week. "If you need to surcharge me for the price of oil, surcharge me. We're kind of disgusted with the folks in Tempe because we're being nickled and dimed."

Terry Trippler, a former travel agent and publisher of a travel-information Web site www.terrytrippler.com, said he expected the airlines that still allowed two free checked bags - American Airlines, Continental Airlines Inc., Delta Air Lines Inc. and Northwest Airlines Corp. - to match US Airways' move by Memorial Day.

The fee is another step in the airlines' efforts to extract more revenue from passengers by charging for services that once were included in the price of a ticket, he said.

"It's a la carte pricing," Trippler said. "You only pay for what you want. I think $1 for a can of soda is just around the corner."

In a message to employees, the airline said the $25 fee would generate about $75 million in additional revenue or cost savings as a way to offset higher fuel prices. The company said it might pay $800 million more for jet fuel this year than it did in 2007 - no small matter, since its entire profit last year was $440 million.

Most discount airlines charge for more than one bag or, in some cases, for all checked luggage. Southwest Airlines earlier this year went from allowing three free checked bags to allowing two.

Some discounters already charge for soft drinks as well.

More common are charges for any on-board food beyond a bag of peanuts or pretzels, $2 apiece for checking bags at curbside at airports, and $25 or more for buying a ticket at an airport, over the phone, or any way other than on the Internet. Airline Web sites normally detail all the charges.

Trippler said he believed airlines "have just about hit the end" of services they can charge extra for, at the same time that they have reduced their nonfuel costs as much as possible.

"There's no more room to cut employees or their salaries," he said. "Now, it's not about cuts but revenue."