The relationship between family functioning and adolescents’ physical aggression has been well established, but whether these relationships might differ by ethnicity has received less attention. Ethnic variations may be important for targeting prevention programs to specific youth and families. This study examined the longitudinal relationship between family cohesion, parental monitoring, and physical aggression using data from the Multisite Violence Prevention Project sample of high-risk youth (elevated aggression). Participants were 1,232 high-risk middle school students (65 % male; 70 % African American; 15 % Hispanic). Meaningful demographic variations were identified. After controlling for intervention condition and study site, family cohesion was significantly negatively related to physical aggression, more so for Hispanic youth. Parental monitoring was negatively associated with physical aggression for African American youth only. Our findings point to the importance of developing culturally sensitive family interventions to prevent physical aggression in middle school.