Southwest Airlines Co. said Friday its traffic last month rose 3.5 percent as the number of available seats remained steady but planes were more full.

Southwest said passenger revenue per available seat mile rose 19 percent to 20 percent from May 2009. That increase far outstripped Southwest’s 3.5 percent traffic increase from the same period.

Continental Airlines also reported this week that its revenue-per-mile figure increased sharply — between 23 percent and 24 percent.

Not all airlines report the figure monthly, but Southwest’s and Continental’s reports indicated that carriers are beginning to boost fares, as well as collect fees for extra services.

In the first five months of the year, traffic rose 1.5 percent to 30.3 billion revenue passenger miles. A revenue passenger mile measures one paying passenger flown one mile.

Capacity so far this year has declined 4.4 percent to 39.46 billion available seat miles. But as Southwest has trimmed its number of available seats, its planes have flown more full. The occupancy rate has increased 4.5 percentage points to 76.8 percent.