Observed 5 male and 19 female infants (mean age 13.4 mo) in infant play groups. Two sets of infant behaviors were coded: assertive acts and attempts to communicate with adults. No sex differences were observed at 13-14 mo in any of these behaviors. However, adults attended to girls’ assertive behaviors far less of the time than to boys’ assertive behaviors. They attended more to girls’ less intense communication attempts and to boys’ more intense attempts. When 29 of the same Ss were observed in toddler play groups no more than 11 mo later, there were sex differences in behavior. Boys were more assertive; girls talked to teachers more. Teachers no longer differentiated their responses to boys and girls. Peers reacted more to boys’ assertive behavior than to girls’. It was hypothesized that caregivers may use stereotypes to guide their reactions to infants because infant behavior is ambiguous. For the toddlers, behavior had become more defined, and caregivers reacted to the behaviors. By using the sex stereotype to guide their reactions to younger children, the caregivers may have perpetuated the stereotype.