Children are increasingly less active and thus are at increased risk for obesity and related diseases. The purpose of this study was to identify social, psychological and environmental determinants of 3-5th grade Hispanic and non-Hispanic white (NHW), and high and low socioeconomic status (SES) children's physical activity. To predict children's physical activity levels we measured: 1) children's perceived athletic competence 2) parent prioritization of activity for themselves 3) parent ranking of the importance of physical activity 4) parent perceived family activity/recreation environment; and 5) parent physical activity. Physical activity was measured using uniaxial accelerometers in parents (51 non-Hispanic white, 48 Hispanic, 48 high SES and 51 low SES families) and their children over 7 days. Each predictor variable was explored for possible interaction between ethnicity and SES. Socioeconomic status had a greater influence on children’s physical activity than ethnicity in this sample. Non-Hispanic white and high SES mothers scored higher on their perceived importance of family activity/recreation environment, prioritization of physical activity for themselves, and were more physically active than Hispanic and low SES mothers. High SES mothers had greater ranking of the importance of physical activity than low SES mothers. There was an interaction between SES and ethnicity across all mothers such that low SES Hispanic and high SES NHW mothers had greater prioritization of physical activity for themselves and total physical activity than high SES Hispanic and low SES NHW mothers. High SES mothers’ valuation of physical activity was greater than low SES mothers. There were no differences in physical activity levels between Hispanic vs. NHW and high vs. low SES children, yet Hispanic and low SES children's BMIs were higher than NHW and high SES children's BMIs. There were no differences between Hispanic vs. NHW fathers' predictor variables nor did any of fathers' data relate to children's physical activity or perceived competence. High SES fathers had greater perceived family activity/recreation environment than low SES fathers. For all groups, children's physical activity was best predicted by children's perceived athletic competence. A re-specified model was developed and tested resulting in mothers’ physical activity and prioritization affecting family’s activity/recreation environment and thus, children’s perceived athletic competence and physical activity. These findings suggest that determinants of physical activity differ between Hispanic and non-Hispanic white families of differing socioeconomic status.Keyword(s): disease prevention, multiculturalism/cultural diversity, physical activity