Southwest said it filled 63% of its seats last month, that's down 6% over last year and the lowest since November 2002.
The carrier, which had been raising fares and avoiding fare sales for much of 2008, launched several fare sales in November in an attempt to boost bookings.
Chairman and CEO Gary Kelly also said Dallas-based Southwest plans to cut capacity early next year, although not quite as sharply as the airline previously thought.
Kelly says they're concerned about the economy.
Southwest earlier reported that the number of paying passengers in November fell nearly 11 percent from a year ago.
The airline carried about 6.5 million passengers last month.
Kelly, speaking to a Credit Suisse investor conference in New York, said Southwest plans to take delivery of 13 new Boeing 737 jets next year.
Southwest already planned to retire three and now must "manage" 10 others -- Kelly didn't specify what it would do with the planes -- to keep the fleet at its current size.