An explanatory model for children’s development of disruptive behavior across the transition from preschool to school was tested. It was hypothesized that child effortful control would mediate the effects of parenting on children’s externalizing behavior and that child sex would moderate these relations. Participants were 241 children (123 boys) and their parents and teachers. Three dimensions of parenting, warm responsiveness, induction, and corporal punishment, were assessed via maternal report when children were 3 years old. Child effortful control at age 3 was measured using laboratory tasks and a mother-report questionnaire. Mothers and teachers contributed ratings of child externalizing behavior at age 6. Results showed that the hypothesized model fit the data well and that the pattern of associations between constructs differed for boys and girls. For boys, parental warm responsiveness and corporal punishment had significant indirect effects on children’s externalizing behavior three years later, mediated by child effortful control. Such relations were not observed for girls. These findings support a sex-differentiated pathway to externalizing behavior across the transition from preschool to school.