In Denver, Frontier seemed vulnerable when Southwest entered its hub two years ago, and that carrier sought bankruptcy protection this year. Now, it's rebuilding. "They definitely haven't killed off Frontier," Boyd says. "Frontier is holding on." In the Minneapolis-Midway market, the hazards are easy to see, because low-cost carrier AirTran ( AAI) dropped the route in May after three years of trying to make it work. Northwest, of course, had made it tough, matching fares and managing schedules to offer better alternatives. Plus, fuel prices were soaring in the spring. Southwest may be better able to remain, Mann says, because of its Midway connections. "Southwest serves 47 cities with 220 flights a day from Midway, and has built that presence continually since 1985," he says.