Another milestone for Southwest Airlines ( LUV).

The airline Tuesday reported strong March traffic figures, including a record load factor for the month.

The low cost carrier said its load factor -- or the number of seats filled -- was 73.6%, up from 67.3% in the year-ago period.

In March, the Dallas-based airline flew 4.7 billion revenue passenger miles, or RPMs, which is one paying customer flown one mile. That measurement of traffic was up more than 15% from last year. Available seat miles, or ASMs, increased 5.4% to 6.4 billion from a year ago.

Southwest also said April and May bookings are "strong in response to recent fare sales."

Southwest shares were up 4 cents at $15.02.

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