Research Findings: This study examined the effects of educators' participation in an in-service training program on the aggressive and prosocial behaviors of preschool-age children. Seventeen early childhood educators were randomly assigned to experimental and control groups. A total of 68 preschool children, 4 from each educator's classroom, also participated. The educator-child play groups included boys and girls. Educators in the experimental group received in-service training on how to facilitate peer interaction; educators in the control group received training on adult-child dyadic interaction strategies. Videotaped interactions of small-group play were coded to capture the frequency of children's use of aggressive and prosocial behaviors. Decreases in aggressive behaviors were found for boys but not for girls. Compared to the control group, the children in the experimental group used significantly more prosocial behaviors following the in-service training. Follow-up studies measuring the stability of decreases in aggression for boys and increased prosocial behaviors have yet to be conducted. Practice or Policy: In-service training for educators that focuses on promoting peer interactions improves children's use of prosocial behaviors during dramatic play in small-group interactions. (Contains 3 tables.)