This study examined interactive relations between adolescent, maternal, and paternal familism values and deviant peer affiliations in predicting adolescent externalizing problems within low-income, Mexican-origin families (N = 598). Adolescent, maternal, and paternal familism values interacted protectively with deviant peer affiliations to predict lower levels of externalizing problems according to two independent teacher reports. These relations were not found with parent reports of adolescent externalizing problems although these models showed a direct, protective effect of maternal familism values. Consistent with the view that traditional cultural values are protective for Latino adolescents, these results suggest that supporting familism values among Mexican-origin groups is a useful avenue for improving adolescent conduct problems, particularly in a school context. (Contains 4 tables, 1 figure, and 1 note.)