The present study aimed to determine the potential moderating effects of temperamental traits on the relation between parenting and toddlers' externalizing behaviours. For that purpose, this study examined the interplay between temperament and maternal parenting behaviours in predicting the level as well as the development of toddlers' externalizing behaviours. Participants were 115 boys (wave 1, M = 16.9 months; wave 2, M = 23.2 months) and their mothers, who were observed in a 13-min structured play session at home. With regard to the prediction of the level of externalizing behaviours at wave 1, main effects were found for children's temperamental characteristics. In addition, maternal negative control interacted significantly with children's inhibitory control in predicting this level of externalizing behaviours. The findings with regard to the development of externalizing behaviours showed that the effects of maternal negative control and lack of maternal sensitivity were stronger for toddlers with a difficult temperament: maternal negative control and lack of maternal sensitivity were related to an increase in externalizing behaviours for temperamentally difficult children only. These results offer support for the goodness-of-fit hypothesis, stressing the idea that the effects of temperament and the social environment depend to a large extent on their interplay. (Contains 4 tables and 2 figures.)