The present longitudinal study examined the reciprocal relationships between teachers' causal attributions and children's math performance and task persistence. In total, 760 elementary school children and their teachers participated in this study. The children were tested in math twice, at the end of the second and third grades. At both time points teachers also filled out questionnaires measuring their causal attributions and assessed children's task persistence. The results indicated that the more the teachers attributed children's math success to their own help or math failure to children's ability, the poorer the children's math performance was. Likewise, the lower the children's math performance was, the more teachers thought that their math success was due to teachers' help or math failure was due to children's ability. In contrast, teachers' perception of success attributions to ability was based on children's task persistence as well as math performance. Persistent behaviour in turn increased children's math performance and vice versa. The findings highlight the importance of teachers' causal attributions in children's learning.