Southwest Airlines Co. said Friday that its number of paying passengers increased at a faster clip than it expanded its capacity in September, making planes more full in a traditionally weak month for air travel.

The company flew paying passengers 7.9 billion miles last month, up 6.4 percent from a year earlier.

Capacity, or the number of available seats, rose 3.2 percent.

The airline's occupancy rate rose to 77.8 percent from 75.5 percent in September 2010.

And the money the airline made from each passenger increased at twice the pace of traffic, because of higher fares. Passenger revenue per available seat mile, a measure of what the airline makes to fly a paying passenger a single mile, rose 12 percent from a year ago.

Last year's figures combine Southwest and AirTran, even though they were then separate companies. The airlines combined on May 2 of this year.

Southwest shares rose 3 cents in morning trading to US$7.89.