This study provides evidence of sex-typed behavior in the spontaneous play of infants between 9 1/2 and 15 1/2 months. Eight boys and eight girls at 9 1/2, 11 1/2, 13 1/2, and 15 1/2 months were observed during 15 minutes of free play with a variety of realistic toys equally divided into male, female, and neutral categories. Stereotypical, relational, and functional play and number of different uses were defined a priori and recorded in 10-second blocks. Significant main effects for age were found for the four classes of play. While total active play remained stable, the four classes of play differed with age. Stereotypical play (85% at 9 1/2 months was replaced by relational play (39%) as the dominant behavior at 13 1/2 months. Functional play, rare at 9 1/2 months, increased to 52% at 15 1/2 months. The number of different appropriate uses increased linearly with age. Significant category x sex interactions were found for stereotypical and functional play. Boys played more with male than female toys. Girls played more with female than male toys. Significant differences in sex related stereotypical play, absent at 9 1/2 months appeared reliably at 11 1/2 months. Significant differences in sex related functional play, absent at 11 1/2 months, appeared reliably at 13 1/2 months. The results indicate that sex type behavior occurs between 9 1/2 and 15 1/2 months and suggest that this phenomenon emerges during this 6-month period. (Author/RH)