Suppressive effect of Ca 2+ on the thermal denaturation of myosin and on myofibrils of squid mantle muscle was compared. Its stabilization effect on myosin was smaller than that on myofibrils, and was not affected by KCl concentration. The stabilizing effect of Ca 2+ on myosin decreased with lowering the temperature for heating showing no stabilization at 20 °C, while the effect on myofibrils was evenly detected at all temperatures tested. Stabilizing effect of Ca 2+ on myosin disappeared even at 30 °C in the presence of sorbitol showing a small inactivation rate, while the effect of Ca 2+ was equally detected on myofibrils irrespective of the reduction of inactivation rate with sorbitol. Stabilization of myosin by Ca 2+ again appeared even in the presence of sorbitol by raising the heating temperature to 38 °C. It was suggested that Ca 2+ confers stabilization on myosin only when myosin is in under unstable conditions. Stabilization effect of Ca 2+ on myosin was enhanced upon F-actin binding, namely Ca 2+ bound myosin was more significant stabilized by F-actin binding, and the effect was no longer affected by the conditions for heating.