Child temperament was examined as a moderator of the link between family conflict and child behavior problems. Temperament assessed in early childhood was used to predict the relation between family conflict and externalizing behavior problems measured during the early elementary school years. For children with difficult temperament, a strong association between subsequent family conflict and behavioral adjustment was predicted; for children with easy or intermediate temperaments, low to moderate associations were predicted. These hypotheses were tested across 3 temperament groups (easy, intermediate, and difficult). Data were collected from 108 children and families participating in the Fullerton Longitudinal Study at ages 3 through 10 years. Consistent with the hypotheses, the data provided support for the moderating role of temperament in predicting the association between family conflict and child externalizing behavior problems. These findings suggest that temperamental difficultness operates as a vulnerability factor with respect to the development of children's behavior problems in families with high conflict.