42 young female subjects aged 19-24 years were selected to form 3 groups based on the estimated hours of physical activity taken each week. They represented 1. very active, 2. active, and 3. sedentary people. Their body fat and fat-free mass (FFM) were estimated from densitometry (corrected for the lung residual volume present during total immersion) and the distribution of subcutaneous fat mass (SFM) was examined by measuring skinfold thickness at 11 sites. The regression of body density on log σ4 skinfolds was significantly different between the 3 groups for a given skinfold value of up to 40 mm, which corresponded to a higher density value for the very active subjects. The possibility that this observation might be explained by differences in the distribution of fat between subcutaneous and internal fat stores was examined, using empirical calculations of SFM. The proportion of fat situated subcutaneously was higher than previously reported (0·65), with a range of individual values (0·41-0·87), but no significant differences in fat distribution between the 3 groups. Group differences in the relationship between body density and log σ4 skinfolds might be explained by variation in the composition of the FFM which may result from differing levels of habitual physical activity. The implication of this possibility in estimating total body fat from density measurements are discussed.