Southwest Airlines Co. said Monday that its passenger revenue in June accelerated while traffic remained virtually flat.

Southwest flew paying passengers 9.62 billion miles last month compared with 9.63 billion miles in June 2011. The airline's capacity, or number of available seats, fell 0.7 percent.

Its passenger revenue per available seat mile, an important measure for airlines because it measures the money they collect from flying one passenger a single mile, rose about 6 percent.

Because the number of available seats fell while traffic remained steady, flights were packed tighter. Southwest's occupancy rate rose 0.5 percentage points to 84.4 percent.

Also Monday, Delta Air Lines Inc. and Southwest finalized a deal where Delta will lease 88 Boeing 717s from Southwest. The deal brings Southwest back to running just one type of plane, the Boeing 737. For Delta, it means fresher planes that are more attractive to higher-paying business travelers and also save on fuel.

Southwest shares slipped a penny to $9.26 in morning trading Monday. They are down 18 percent since peaking at $11.28 almost a year ago.