Transverse sections of the semimembranosus muscle of a control (C) line of Japanese quail and of a line (P) selected for high body weight at 4 weeks of age were observed for nuclear content. The two lines were sampled at 4 days posthatching and at 4 weeks of age. The relative number of satellite cells decreased from 4 days to 4 weeks of age. The total number of satellite cells, however, appeared to increase as muscle length increased; this relation was not influenced by line effects. It was concluded that the greater number of nuclei in the longer fibers of P-line quail was principally the consequence of a greater mitotic rate in the satellite cells of this line compared to the C-line quail.