Damian Miller is the most recent player I can remember who wasn't part of the MLBPA. Players do not have to join the MLBPA. According to the MLBPA website: Q: Who is eligible for membership in the Association? A: All players, managers, coaches and trainers who hold a signed contract with a Major League club are eligible for membership in the Association. In collective bargaining, the Association represents around 1,200 players, or the number of players on each club's 40-man roster, in addition to any players on the disabled list. Q: How much are union dues? A: The players' dues are $85 per day during the season. http://www.mlbplayers.com/ViewArticle.dbml?&DB_OEM_ID=34000&ATCLID=211785603