Background: Inconsistencies regarding developmental effects of non-maternal childcare may be caused by neglecting the possibility that children are differentially susceptible towards such experiences. Method: Interactions between difficult/negative child temperament and childcare type, quantity, and quality on teacher-rated behavior problems and social competence at 54 months and in kindergarten were investigated via multiple regression using data from the NICHD Study of Early Child Care. Results: Childcare quality interacted with infant negativity in predicting behavior problems and social competence, whereas effects of quantity and type were independent of child temperament. Consistent with Belsky's (1997) differential susceptibility hypothesis, children with difficult temperaments as infants exhibited both more behavior problems when faced with low quality care and fewer when experiencing high quality care than children with easy temperaments. Conclusions: Negatively-emotional infants appear to be more affected by the quality of care they experience--both negatively and positively--than other young children.