The failure of developmental tests in infancy to predict later competence may be because the tests ignore important dimensions of infant functioning. Yarrow and Pedersen (1976) have suggested that one such dimension could be mastery behavior, which involves persistence and investigation. Fifty-three infants were observed at 6 and 12 months during two 24-min play sessions. The Bayley Scales of Infant Development were given at 6 and 12 months, and the McCarthy Scales of Children's Abilities at 30 months. Measures of competence in infancy (successful task completion during play and the Bayley scores) were not strongly correlated with the 30-month McCarthy Scales. In contrast, infant mastery behavior during play strongly predicted McCarthy scores: The time spent investigating toys at 6 months, and persistence in solving tasks at 12 months, were behaviors significantly positively correlated with the McCarthy Scales. Thus, infant behaviors that predict later competence do not remain static, but change with age. More importantly, infants' mastery behavior appears to be a more effective predictor of later development than their competence with either toys or developmental tests.