We examined the criterion validity of parent and self-report versions of the Junior Temperament and Character Inventory (JTCI) in children with high levels of externalizing problems. The sample included 412 children (206 participants and 206 siblings) participating in a family study of attention and aggressive behavior problems. Criterion validity analyses included (a) correlations between temperament scales and emotional and behavioral scales and (b) correlations between temperament and intelligence and achievement scales. Temperament scales displayed strong convergent and discriminant validity. Across informants and samples, JTCI scales assessing novelty seeking and harm avoidance discriminated between internalizing and externalizing problems. Reward dependence, persistence, cooperativeness, and self-directedness displayed similar patterns of negative relations to emotional and behavioral scales and positive relations to intelligence, achievement, and competence.