Purpose: It is unclear how the amount of ambulatory activity (AA) participated in by older adults relates to body composition or leg strength. The aim of this study was to describe associations of pedometer-determined AA with body fat and leg muscle parameters in community-dwelling 50- to 79-yr-olds. Methods: A cross-sectional study of 982 randomly recruited subjects was conducted (51% female; mean age = 62 ± 7 yr). Dual-energy x-ray absorptiometry measured body composition, including total body fat, trunk fat, and leg lean mass. Isometric strength of the quadriceps and hip flexors was measured using a dynamometer. Leg muscle quality was calculated as kilograms of leg strength per kilogram of leg lean mass. Individual AA was recorded over seven d using a pedometer. Results: Average AA was 9622 ± 4004 steps per day. There was no evidence of a threshold model between AA and body fat, leg lean mass, or leg strength. Multivariable regression analyses adjusting for age revealed that AA was negatively associated with total body fat (overall [beta] = -0.54, P < 0.001; partial R2 = 0.06) and trunk fat mass (overall [beta] = -0.28, P < 0.001; partial R2 = 0.05). In women only, a significant positive association between AA and both leg strength ([beta] = 0.71, P = 0.016; partial R2 = 0.01) and leg muscle quality ([beta] = 0.08, P = 0.001; partial R2 = 0.02) was observed. Conclusions: These results suggest that pedometer-determined AA is a major determinant of body fat in community-dwelling older adults and is also involved in the maintenance of leg strength and muscle quality in older women.