High vitamin D and physical activity (PA) levels are independently associated with improved body composition and muscle function in older adults. OBJECTIVE: The objective of this study was to investigate the interaction of 25-hydroxyvitamin D (25OHD) and PA status in maintenance of body composition and muscle function in older adults. DESIGN AND SETTING: This was a 5-year prospective population-based study of Australian community-dwelling older adults. PARTICIPANTS: PARTICIPANTS in the study included 615 community-dwelling volunteers aged 50 years old or older [61.4 ? 6.9 (mean ? SD) y; 48 female] randomly selected from electoral rolls and categorized according to baseline serum 25OHD (= or .05). Among participants with accelerometer data, these associations were generally mediated by higher levels of moderate/vigorous PA. CONCLUSIONS: High vitamin D status appears to enhance PA-related declines in body fat during aging, but the mechanism may be greater amounts of outdoor moderate/vigorous PA rather than a direct effect of 25OHD.