Interactions between reactive and regulatory dimensions of temperament may be particularly relevant to children's adjustment but are examined infrequently. This study investigated these interactions by examining effortful control as a moderator of the relations of fear and frustration reactivity to children's social competence, internalizing, and externalizing problems. Participants included 306 three-year-old children and their mothers. Children's effortful control was measured using observational measures, and reactivity was assessed with both observational and mother-reported measures. Mothers reported on children's adjustment. Significant interactions indicated that children with higher mother-reported fear or higher observed frustration and lower executive control showed higher externalizing problems whereas children with higher observed fear and higher delay ability demonstrated lower externalizing problems. These results highlight effortful control as a moderator of the relation between reactivity and adjustment, and may inform the development of interventions geared toward the management of specific negative affects. (Contains 3 figures and 4 tables.)