Previous studies have implicated acculturation to the US as a risk factor for unhealthy behaviors among Hispanic and Asian-American adolescents, including substance use, violence, and unsafe sex. This study examined the association between acculturation and obesity-related behaviors-physical activity and fast-food consumption-among 619 Asian-American and 1385 Hispanic adolescents in Southern California. Respondents completed surveys in 6th and 7th grade. The 6th grade survey assessed acculturation with the AHIMSA acculturation scale and a measure of English language usage. The 7th grade survey assessed frequency of moderate-to-intense physical activity and frequency of eating fast-food. Multiple regression analyses included acculturation and demographic covariates as predictors of physical activity and fast-food consumption. Acculturation to the US, assessed in 6th grade, was significantly associated with a lower frequency of physical activity participation and a higher frequency of fast-food consumption in 7th grade. The significant associations persisted after controlling for covariates and were consistent across gender and ethnic groups. Results suggest that acculturation to the US is a risk factor for obesity-related behaviors among Asian-American and Hispanic adolescents. Health promotion programs are needed to encourage physical activity and healthy diets among adolescents in acculturating families.