A study was conducted to test the hypothesis that spontaneous mastery would predict infant functioning on the Bayley Mental Development Index better than would infants' executive capacity scores. At 12 and 18 months of age, 38 infants (20 from Pennsylvania and 18 from Bermuda) were observed for 30 minutes in their homes while engaging in spontaneous play with two sets of toys. Each child's most sophisticated play act was coded by the observer on a 12-step play scale. (The 12-step play scale reflects the progression of play behaviors from the infants' early explorations and object groupings to their later emerging pretend acts directed at both self and others, their substitution of objects, and their use of sequenced acts in pretense.) To measure infants' executive capacities, the same observer encouraged the child to engage in play more sophisticated than that which was displayed during free play. The child's executive capacity scores were then established as the difference between spontaneous and elicited play. Infants' mastery persistence scores were calculated by weighting the frequency of spontaneous play activity according to the play level in which it occurred. Results provided support for the study's hypothesis. (MP)