Southwest Airlines mechanics rejected a plan for combining their seniority list with that of mechanics at AirTran Airways, complicating Southwest's job of combining the two airlines.

The dispute could go to arbitration, although Southwest on Tuesday held out the hope that the two labor groups could come together on their own.

Seniority often determines job assignments and pay in the airline industry.

Members of Local 11 of the Aircraft Mechanics Fraternal Association, which represents about 1,600 Southwest mechanics, voted against combining the seniority lists.

Voting ended Tuesday. Union officials did not immediately return messages seeking comment.

Southwest said the next step would be a filing for arbitration by AMFA and the Teamsters, which represents about 400 AirTran mechanics.

Southwest Airlines Co. bought AirTran for US$1.4 billion last year.

Pilots and flight attendants at the two airlines have approved seniority-combination agreements. Several groups of ground workers are still negotiating.