In this study, 434 children and adolescents completed the Control-, Agency-, and Means-ends beliefs Interview (CAMI). Moreover, we asked for current mood-state and achievement-related affect after success and failure at school. There was no gender difference in achievement-related affective expression in children (9 and 10 years old), but there was a substantial difference in adolescents (15 and 16 years old), boys reporting less affect than girls. A different pattern emerged for the impact of affect on achievement-related beliefs. Again, children did not show any gender difference. In adolescents, however, boys' current mood-state was related to agency beliefs to a higher extent than girls' current mood state. Thus, boys, in comparison to girls, may develop inhibition of affective expression, but over the same span, they continue to base agency beliefs on their current mood-state whereas girls do not. (Contains 8 tables, 3 figures, and 4 notes.)