Habitual physical activity in children is related to physical fitness and appears to mediate cardiovascular disease (CVD) risk factors. We studied the physical activity patterns and associated variables of a large bi-ethnic cohort of 4-year-old children from low to middle socioeconomic families. Trained observers coded the behavior of 351 children (150 Anglo-American, 201 Mexican-American; 182 boys, 169 girls) during two 60-minute home visits and two unstructured recesses lasting up to 30 minutes each at 63 different preschools, Findings indicated that although children were much less active at home, there were low but significant correlations between their activity patterns at home and during recess (r = .13). Children who had activity-promoting toys at home also tended to have them available during preschool recess (r = .20). Ethnic differences were evident for both activity and environmental variables. Mexican-American children were less active than Anglo children at home (p < .002) and during recess (p < .03), thus adding to the adult literature that has found Mexican-Americans to be less active than Anglos, and supporting to the notion that physical activity life-style habits may be established in early childhood. In both settings, Mexican-American children spent more time in presence of adults (home, p < .04; recess, p < .03) and had access to fewer active toys (home, p < .001; recess, p < .05). Gender differences were also evident for both activity and environmental variables. Boys were more active both at home (p < .01) and during recess (p < .01) and at home watched television more frequently (p < .005). These data indicated some subgroups of preschool children are more active than others, and the findings may be useful for pediatricians and educators responsible for developing physical activity promotion programs for young children. J Dev Behav Pediatr 13:173–180, 1992. Index terms: preschool children, physical activity, systematic observation.