Southwest spokesman Chris Mainz said Tuesday that the Philadelphia-Boston route "just hasn't performed the way that we would have liked."
"With the current economic environment, fuel prices, we are having to make some hard decisions with regard to flights and routes," he said.
Southwest has been retreating in Philadelphia, from a peak of 71 flights to 20 cities, to 55 flights to 18 destinations currently.
In July, Southwest announced it would eliminate four of those destinations on Jan. 8: Pittsburgh; Providence, R.I.; Manchester, N.H.; and Jacksonville, Fla.
At the time, Southwest also said it would trim Philadelphia-to-Boston flights from eight a day to six, then to five in February. But there was no mention of its pulling out of the route altogether.
The Dallas-based low-cost airline - which recently acquired the smaller AirTran Airways - is expanding in cities including Atlanta and Denver. In Philadelphia, US Airways, with an international hub and 444 daily flights, has proved to be a tough competitor, with fares that often beat Southwest's ticket prices.
"Obviously, Southwest sees a bunch of opportunities around the AirTran acquisition," said airline analyst Bob McAdoo, of Avondale Partners L.L.C.
The competitive situation in Denver, where United Airlines is pulling back, and in Atlanta, where AirTran is strong, may look "more attractive than continuing to grind out marginal returns in Philadelphia against US Airways, which has been a very strong, very dedicated competitor," McAdoo said.
He speculated that low-fare JetBlue Airways, with 100 daily flights from Boston - that city's largest carrier - may now come here.
"JetBlue's target is to get up to 150 flights a day and to be the airline the Boston businessperson calls when they want to go somewhere on business," McAdoo said. "With this announcement by Southwest, I would wager that JetBlue is going to be in Philadelphia."
US Airways can make money on the Boston- and Pittsburgh-to-Philadelphia routes, where Southwest may not, because it takes passengers from the Philadelphia hub on to many other destinations and does not depend only on traffic beginning and ending journeys in those cities.
Southwest, whose entry here in 2004 invigorated air travel with friendly service and lower fares, began with five daily round-trips to Boston and an introductory one-way fare as low as $59.
For consumers, the advantage of competition is lower fares. When US Airways had a monopoly on the Boston route, 280 air miles, fares were as high as $550 one-way for a nonstop coach seat, or $1,100 round-trip.
US Airways matched Southwest's fares, and on Tuesday the lowest nonrefundable fare on both the Southwest and US Airways websites was $39 one-way, if purchased in advance.
A check of US Airways' website for Feb. 15 and April 12 on Tuesday still showed a nonrefundable $78 round-trip fare from Philadelphia to Boston. How long that fare will remain is unclear.
After AirTran stopped flying the Philadelphia-Boston route in 2007, ticket prices crept up. In August 2009, Delta Air Lines Inc. also dropped the route.
Contact staff writer Linda Loyd at 215-854-2831 or firstname.lastname@example.org.