Few data on physical activity habits among populations of low socioeconomic status have been published. The authors studied physical activity habits--leisure-time physical activity, job-related physical activity, household physical activity, and walking--among 172 lower socioeconomic status women and 84 lower socioeconomic status men and compared their habits with those of 208 higher socioeconomic status women and 95 higher socioeconomic status men. All subjects resided in the greater Pittsburgh, Pennsylvania, area. Data collection occurred throughout 1986. Lower socioeconomic status women, the least active group, averaged 1,536 +/- 1,701 minutes/week (+/- standard deviation) of total physical activity, whereas higher socioeconomic status women, the most active group, averaged 2,079 +/- 1,807 minutes/week (p less than 0.0001). Higher socioeconomic status men averaged 1,952 +/- 1,799 minutes/week, and lower socioeconomic status men averaged 1,948 +/- 1,916 minutes/week. Higher socioeconomic status women spent significantly more time each week in leisure-time physical activity, job-related physical activity, and household physical activity than did lower socioeconomic status women. Lower socioeconomic status men spent significantly more time each week walking and doing household chores, whereas higher socioeconomic status men tended to be more active in leisure-time physical activity. These data suggest important quantitative and qualitative differences in physical activity among population subgroups. In view of the important role of physical activity in promoting physical and mental health, reasons for the differences among groups of varying socioeconomic status must be examined and elucidated.