This study examined the development of imitation under naturalistic conditions. Participating were 320 parents, who provided diary records of imitation by their 12-, 15-, and 18-month-old infants over a 7-day period. Approximately half of the infants were first-borns and half had older siblings. The findings indicated that infants of all ages acquired one to two new behaviors per day by imitation. There were age-related changes in the quality rather than the quantity of behaviors acquired through imitation. Older infants imitated more multi-step sequences and substituted more objects during reenactment than did younger infants. There were also sibling-related changes in the quality of behaviors acquired through imitation. Infants with siblings imitated more behaviors spontaneously and their imitation was characterized by a higher level of pretense than infants without siblings. The findings were highly consistent with those obtained under more controlled laboratory conditions. The similarity of the two sets of findings increases the validity of current laboratory research on imitation and indicates that imitation is a powerful mechanism by which infants acquire new behaviors in the course of their everyday lives. (Author/AMC)