Changes in manipulative play with objects were examined in a longitudinal sample of 10 boys and 9 girls tested at ages 9, 13, and 18 months. Stability of individual differences in play was also examined. Each child was observed individually for 7 minutes in a room in which a tea set was the only toy present. Seven types of play behavior were scored: banging, mouthing, simple relational acts, accommodative relational acts, symbolic acts, and sequential acts. Clear differences in play were apparent between 9- and 13-month-olds. Generalized motoric schemes declined, while exploration of physical and functional relations between objects increased. In addition, symbolic play appeared for most children at 13 months of age. Further developments in types of symbolic play were present at 18 months. Infants demonstrating an early head start in maturity of play at 9 months tended to maintain this advantage through 13 months. It is suggested that since children's actions identify newly emergent cognitive themes, observations of play can provide valuable insights into the course of cognitive growth. (Author/CM)