DALLAS (AP) -- Southwest Airlines Co. says that passenger traffic is rising but a key revenue measure continues to fall.
The company, which also operates AirTran Airways, said Friday that passenger revenue for every seat flown one mile dropped about 2 percent in May compared with the same month last year.
That's a closely watched figure in the airline business. The revenue ratio rises if airlines fill more seats and sell more higher-priced fares.
In April the same figure declined by between 4 percent and 5 percent at Southwest and also fell at American, US Airways and Delta Air Lines.
Although revenue per seat sagged, Southwest and AirTran had no trouble filling seats in May. The company said that passengers flew 9.35 billion miles, up 4.2 percent from May 2012.
Traffic rose faster than Southwest and AirTran added seats — they boosted capacity by 3.4 percent. As a result, the average flight was 81.9 percent full, up slightly from 81.3 percent a year earlier.
Dallas-based Southwest is the nation's fourth-biggest airline by passenger miles.
Southwest shares rose 35 cents, or 2.6 percent, to $13.84 in midday trading.