Male and female chicks of Rhode Island Red with normal growth were used for this study. Twenty-four individual muscles were weighed on the chicks' right. Postnatal growth of individual muscles were compared with each other, and its sexual differences were studied. The results were as follows: 1. In I-period (0-2 weeks old), individual muscles of the forelimbs' part, for example, M. pectoralis superficialis, M. pectoralis profundus and M. triceps brachii, grew and developed very rapidly, but M. biceps femoris and M. semimembranosus intermediately, and the other muscles more slowly than those muscles in both sexes. 2. In II-period (2-22 weeks old), individual muscles of the hind legs' part, for example, M. peroneus longus, M. semimembranosus, M. biceps femoris and M. triceps surae, grew and developed a little more rapidly than the other in the male. And M. peroneus longus, M. biceps femoris, M. semimembranosus and M. pectoralis profundus, etc., grew and developed a little more rapidly than the other in the female. 3. In III-period (22-26 weeks old), individual muscles of the hind legs' part, for example, M. peroneus longus, M. biceps femoris, M. semimembranosus and M. semitendinosus and so on, grew and developed very rapidly in the male. But in the female, all the individual muscles did not grow and develop, or did only a little. 4. In IV-period (after 26 weeks old), individual muscle weights of the 32 weeks old were little heavier than those of the 26 weeks old in the male. 5. From these results mentioned above, individual muscles were divided into six groups as follows: The first A-group (M. pectoralis profundus, M. coracobrachialis externus and M. teres major et infraspinatus): These muscles grew and developed very rapidly during I-period, and rapidly during II- and III-period also. But the rate of growth of these muscles during II- and IIIperiod was similar to that of the third C-group in the male, because these did not show any sexual characteristics of the male. The first B-group (M. pectoralis superficialis, M. latissimus dorsi, M. deltoideus, M. triceps brachii, M. biceps brechii and M. extensor carpi radialis brevis): These muscles grew and developed very rapidly during I-period like the first A-group. But during II- and III-period these grew and developed more slowly than the first A-group. The rate of growth and development of these during II- and III-period was worst of all groups in the male, because these did not show any sexual characteristics of the male. The second group (M. biceps femoris and M. semimembranosus): These muscles grew and developed intermediately during I-period, and very rapidly during II- and III-period. Particularly in the male these made remarkable progress during II- and III-period. The third A-group (M. semitendinosus, M. quadriceps femoris, M. gracilis, M. triceps surae and M. peroneus longus): These muscles grew and developed more slowly during I-period, but very rapidly during II- and III-period. Particularly in the male these made remarkable progress during II- and III-period like the second group. The third B-group (M. gluteus medius, M. gluteus superficialis, M. tensor fasciae latae and M. tibialis cranialis): These muscles grew and developed more slowly during I-period. The rate of growth of these during II- and III-period was similar to that of the first B-group in the female. But in the male, these grew and developed intermediately during II- and III-period, for the rate of growth of these was better than the first A-group, or worse than the second group and the third A-group. The third C-group (M. semispinalis cervicis, M. longus colli, M. sartorius and Mm. adductores): These muscles grew and developed more slowly through all the experimental period. But in the male, the rate of growth of these during II- and III-period was alike the first A-group owing to the sexual characteristics of these muscles.