DALLAS —

If you're taking a nonstop flight to summer vacation, better pony up a lot more money or start unpacking.

In many cases, major carriers have more than doubled or even tripled their cheapest U.S. fares from last summer's fares. That's on top of the new fees for checking luggage and other services.

Tom Parsons, chief executive of the discount travel site Bestfares.com, looked at the lowest fares for nonstop travel in July - the kind of tickets that usually must be bought long in advance and therefore appeal mostly to vacationers, not business travelers.

Parsons said where major carriers offer nonstop flights and low-fare competitors offer only one-stop service, fares are up to 365 percent higher than a year ago.

Last summer, Parsons said, American Airlines offered fares as low as $238 for July travel between Phoenix and Miami. This year, the lowest fare is $660, he said.

Delta's lowest Atlanta-Tampa nonstop round trip starts at $392 for the week of July 22, compared with $148 last year, Parsons said. And United's best deal for Chicago-Charleston, S.C., is $730, up from $178 a year ago, he said.

In all the cases, Parsons said, low-cost carriers such as Southwest Airlines Co. offer only one-stop or slower service.

"You better look for alternate airports or one-stop service because airlines are betting that they can get away with these fares," he said.

The major airlines have raised fares about a dozen times since late last year to offset skyrocketing costs for jet fuel, which has nearly doubled in price over the past year, pushing most airlines into the red.

The older, so-called legacy carriers are also adding or increasing fees for things such as checking a second piece of luggage, changing flights and putting a pet on board. This month, American will begin charging $15 for the first checked bag.

Officials for the major airlines did not dispute Parsons' figures but explained that they must raise fares to cover higher costs, especially fuel.

David Castelveter, a spokesman for the Air Transport Association, an airline industry trade group, said average fares are less than 1 percent lower than they were in 2000 but the price of jet fuel has increased 217 percent in the same time. He said the carriers were "playing catch-up."

"Our sense is that almost all fares are higher now since airlines have raised fares and fuel surcharges because of the massive run-up in jet fuel prices over the last year," said Tim Wagner, a spokesman for American. "Where fares may not have risen as much, or proportionately, the reason is simply going to be because we are competing with carriers that have not raised their fares at the same rate."

Wagner said the fares that Parsons cited continue into September, after the peak travel season, "so it's not a summer phenomenon."

Parsons said, however, that airlines will think twice if they see soft bookings in late July and August for the typically slower fall season. "As we get to the end of summer, I expect most of these fares to fall," he said.

Airlines have historically charged a little more for nonstop flights. Robin Urbanski, a spokeswoman for UAL Corp.'s United Airlines, said some travelers buy tickets on price alone, while other consider schedule, convenience and comfort.