The contribution of selected physical development and muscular strength measures to general motor performance capacity was investigated in 55 seven to twelve year old boys. The physical development variables were height, body weight and fat-free body. Right and left hand strength, and static leg extension force of the right, left and both legs. simultaneously were used as measures of strength. The measures of general motor capacity were six field performance tests of the kind typically used by investigators and teachers including the softball throw, vertical jump, standing broad jump, shuttle run, 50 yard dash, and mile run. Data were analyzed by the method of least squares and constants fitted for height, body weight, fat-free weight and various measures of strength with chronological age held constant. Strength accounted for 18 to 43 percent of the variation in the children's motor performance score. Two measures of physical development, body weight and fat-free body weight together with age, were the dominant factors, contributing from 46 to 65 percent of the variation. Muscular strength and physical development measures together accounted for 57 to 70 percent of the variability in motor performance. For diagnostic and research purposes, measurement of physical development is an important consideration for proper interpretation of field tests of motor performance.