WE have reported determinations of the actin and myosin in the skeletal muscle of a chick 721,2 and 963 hr. after incubation respectively. In order to test whether the appearance of actin in the developing skeletal muscle before the formation of myosin is limited to chick embryos or is a general phenomenon in animals I investigated the formation of actin and myosin in the embryonic stages, and in regenerating limbs after amputation at the knee, of Triturus pyrrhogaster using the same methods as before.