This study examined infants' negative emotionality as moderating the effect of parent-child mutually responsive orientation (MRO) on children's self-regulation (n=102). Negative emotionality was observed in anger-eliciting episodes and in interactions with parents at 7months. MRO was coded in naturalistic interactions at 15months. Self-regulation was measured at 25months in effortful control battery and as self-regulated compliance to parental requests and prohibitions. Negative emotionality moderated the effects of mother-child, but not father-child, MRO. Highly negative infants were less self-regulated when they were in unresponsive relationships (low MRO), but more self-regulated when in responsive relationships (high MRO). For infants not prone to negative emotionality, there was no link between MRO and self-regulation. The "regions of significance" analysis supported the differential susceptibility model not the diathesis-stress model.