On studying the steady-state activity in 0.6M KCl, it was found that Mg-ATPase of chicken gizzard myosin was identical with that of rabbit skeletal myosin in the pH-activity profile, Michaelis-Menten constant, and maximum velocity. As regards the “initial burst” of ATP splitting in the presence of Mg (0.6M KCl), it was found that gizzard and skeletal myosins were identical both in the size of the initial burst and in the velocity-ATP concentration relationship. The only difference we observed was that the Ca- and EDTA-ATPase activities of gizzard myosin were, as reported by other investigators, approximately one-half to one-third of those of skeletal myosin, although the pH-activity profile for the ATPase of gizzard myosin was essentially the same as that of skeletal myosin.