This study investigated whether the parenting stress-child externalizing behavior link is moderated by children's emotional reactivity, as indexed by skin conductance responses (SCRs). Participants were 61 children aged 9-12 years and their mothers. Mothers completed measures of parenting stress and their children's externalizing symptoms; children also reported on their externalizing behavior. Children's SCRs were assessed during the viewing of standardized pleasant, unpleasant, and neutral pictures. Cluster analysis on SCRs identified two groups, labeled Lower SCRs and Higher SCRs. Regression analyses indicated that among children with lower SCRs, those exposed to increased parenting stress reported more externalizing symptoms, whereas those who experienced low parenting stress reported similar rates of externalizing problems as children with higher SCRs. No effect of parenting stress emerged for children with higher SCRs. Findings suggest that higher parenting stress renders children with lower, as opposed to higher, SCRs to emotional stimuli more vulnerable to externalizing problems.