The two dominant labor market turnover hypotheses, the firm-specific human capital model (FSHCM) and the job-matching model, suggest different patterns of player mobility in major league baseball. The matching hypothesis predicts greater mobility of players in positions that require substantial team production. A better match may offer large productivity gains. Alternately, the FSHCM predicts players in positions requiring the greatest amount of teamwork will benefit from specific knowledge, making them less likely to change teams. We examine the frequency distribution of trades by player position from 1900–1992 and find the FSHCM provides the best explanation for turnover in this industry.