The purpose of this study was to assess the interactive effects of habitual physical activity (total and vigorous intensity) and calcium intake on bone mineral content (BMC) in prepubertal boys and girls. Seventy-six children, aged 8–11 yr, wore accelerometers for up to 7 days to assess activity. Calcium intake was estimated by a 4-day weighted food diary. BMC and areal density (bone mineral density) were measured at the total body, proximal femur, and femoral neck by using dual-energy X-ray absorptiometry. Moderated regression analyses were used to assess the contributions of physical activity (total and vigorous) and calcium intake to BMC, residualized for bone area and body mass. Interactive effects of vigorous activity (≥6 metabolic equivalents) and calcium intake were found at the total body in boys (b = 2.90 × 10−3) and in girls (b = 6.58 × 10−3) and at the proximal femur (b = 9.87 × 10−5) and femoral neck (b = 2.29 × 10−5; where b is the regression coefficient from final equation) in boys only; residualized BMC was high only if both vigorous activity and calcium intake were high. There were no interactive effects of total activity and calcium intake. This study provides evidence for synergistic action of habitual vigorous activity and calcium intake on bone mass in children. Recommendations for optimizing bone mass should reflect this synergism.