Regular physical activity is recommended for health maintenance in adolescence, but basic descriptive epidemiological data are lacking for this age group. The present study examined socioeconomic status (SES), ethnic, and sex differences in physical activity in a multiethnic sample of 1871 high school students in San Diego, California. Surveys were completed in required classes by a diverse sample of students from a low-income school district and by Anglo students from a nearby more affluent district. Boys reported more vigorous exercise outside of school and during school physical education, as well as more participation in sports teams, but girls reported taking more activity-related lessons and classes. High-SES students had more frequent physical education classes, spent more time in vigorous exercise in those classes, and participated in more activity lessons outside of school. There were few ethnic differences on summary physical activity or physical education variables. There were no SES differences, but there were ethnic differences on 5 of 22 specific activities. Demographic differences in 25 psychological, social, and environmental variables that may influence physical activity were also examined. Differences between boys and girls were found on several variables, which may explain some of the boys' higher activity levels. Ethnic and/or SES differences were found in 10 of 25 potential correlates of physical activity. These data may be used to identify specific activities that may be preferred by subgroups of adolescents and specific mediating variables that can be targeted in physical activity promotion programs for adolescents.