The paper reports on a study which tested whether infants high in negative affectivity are differentially susceptible to observed coparenting behavior in relation to their subsequent social-emotional development. Data came from a longitudinal study of 182 US dual-earner, primiparous couples and their infant children. At nine-months postpartum, child negative affectivity was reported by mothers and fathers and supportive and undermining coparenting behavior were assessed from mother-father-infant observations. At 27-months mothers reported on toddlers' externalizing behavior and dysregulation using a clinical assessment tool designed to identify competencies and areas of concern in toddlers' social-emotional development. Hierarchical regression analyses revealed partial support for the differential susceptibility hypothesis. Specifically, infants high in negative affectivity had lower levels of dysregulation when embedded in a more supportive coparenting context, and higher levels of dysregulation when embedded in a less supportive coparenting context. In contrast, supportive coparenting behavior was not relevant for the dysregulation of infants initially low in negative affectivity.