Male and female chicks of Rhode Island Red with normal growth were used for this study. We studied what kind of relation between the rate of growth of skeletal muscle and those of several other tissues there were and whether there were sexual differences related to the growth of skeletal muscle or not. The living and the dressing body weights were measured. Moreover, the tissues of skin, viscera, abdominal fat, total bone, total fat and total muscle were weighed respectively. The results obtained were as follows: 1. The living and the dressing body weights of the female were heavier than those of the male at birth, but after 1 week those of the male were always heavier than the other. There were obvious sexual differences in the rate of growth of the living and the dressing body weights after 1 week. Percentages of the dressing body weight to the living one grew larger rapidly from hatching to 2-3 weeks old in both male and female. Thereafter those were about constant, but during sex-maturing period that of the female grew more than the other. 2. When the rate of growth in the weights of several tissues was compared with each other as an increase rate of weights of the birth, the results at 26 weeks old were as follows: In the male, these became larger in the following order : 37.7 times of viscera, 95.6 times of total bone, 1.37 times of skin, 150 times of total fat and 227 times of total muscle, and in the female, 44.6 times of viscera, 47.0 times of total bone, 78.8 times of skin, 125 times of total muscle and 210 times of total fat. From these results mentioned above, we suppose the skeletal muscle is the late-maturing tissue. Because the abdominal fat was weighed primarily at 2 weeks old, the increase rate of it was not compared with the other directly. But, the abdominal fat of the male and the female increased 84.1 times and 134 times respectively between 2 and 26 weeks old. These results indicate the abdominal fat is also late-maturing. 3. The total muscle weight of the male was about 28% of dressing body weight during the first 1 week. It increased gradually to 51.1% at 22 weeks old, and 57.2% at 26 weeks old. On the other hand, those of the female increased from about 25-27% of dressing body weight at hatching or 1 week old to 49.2% at 22 weeks old, but decreased to 43.3% at 26 weeks old conversely. This sexual difference is an interesting consequence.