The purpose of this research was to examine the role that young children's same-sex peer interactions play in influencing early school competence. The authors also examined the degree to which effortful control (EC) moderated these relations. The same-sex play preferences of 98 young children (50 boys and 48 girls; mean age = 54.77 months) were observed during the fall semester. At the end of the fall semester, one set of teachers reported on children's EC, and at the end of the following springsemester, another set reported on children's school competence (social, academic, and perceptual-motor). Results revealed that EC moderated the relations of children's same-sex play to their school competence. These patterns differed for boys and girls such that same-sex play was positively related to school outcomes for boys high in EC and for girls low in EC.