High levels of teacher–child conflict have repeatedly been found to amplify children's aggressive behaviour. Up to now, however, research on possible mechanisms explaining this link is largely lacking. The current study aimed to test whether children's self-esteem is an intervening mechanism. Participants were 139 children (70 boys, M age = 6.18 years) and their teachers from 35 classes in 19 schools who were followed longitudinally throughout first grade. Teacher–child conflict was measured during the first trimester (October to December), children's self-esteem (three child interviews) during the second trimester (January to March) and children's aggressive behaviour during the first and third trimester (April to June). Teacher–child conflict had a significant indirect effect on changes in children's aggressive behaviour across first grade through its effect on children's self-esteem. Copyright © 2011 John Wiley & Sons, Ltd.