Major League Soccer's referees and their assistants are unionizing.

The match officials said Monday they had voted to certify the Professional Soccer Referees Association (PSRA) as their collective bargaining agent in a vote conducted under the rules of the National Labor Relations Board.

Referees, linesman and fourth officials are included in the bargaining unit.

The Professional Referee Organization (PRO), responsible for managing match officials for MLS, will be required to bargain with PSRA over the terms and conditions of employment.

Lucas Middlebrook, a lawyer for PSRA, said the vote was 55-7 among 77 eligible voters.

He said some referees had annual contracts for 2013 but that many referees and all assistants and fourth officials did not.