We investigated the longitudinal relations between parents' (T1 n = 582, T2 n = 564) and teachers' (T1 n = 39, T2 n = 39) beliefs about children's (T1 n = 582, T2 n = 576) competence at primary school grade 3 (T1) and the subsequent interest and self-concept of ability of children during grade 4 (T2) in literacy and math, using longitudinal cross-lagged panel models. Furthermore, we explored the interactive effects of parents' and teachers' beliefs on children's subsequent interest and self-concept of ability. While parental beliefs did not predict children's interest and self-concept in either subject, teacher beliefs predicted children's self-concept, but not their interest, in both literacy and math. Parent and teacher beliefs also showed a significant interaction effect with parental beliefs positively predicting self-concept in both subjects when teacher beliefs were high. When teacher beliefs were low, parent beliefs were not associated with children's self-concepts.