Preschoolers' (60 boys and 64 girls, "M" age = 50.73 months) affiliations with prosocial peers were observed in naturally occurring interactions and then examined in relation to positive and negative emotionality within their peer interactions one semester later. Greater affiliation with prosocial peers in the fall was related to enhanced positive emotionality (especially for girls) and decreased negative emotionality (especially for boys) in later peer interactions. These findings held when initial levels of the emotion were controlled and were found above and beyond variations in classroom levels of prosociality. The findings suggest that peers exert influence in early childhood and that these effects can positively affect the quality of young children's later peer interactions. (Contains 4 tables.)