When rabbit myosin is treated at 35°C, decreases in the ATP ase activity of actomyoshin and in its superprecipittion ability (as well as an increase in teh turbidity of myosin suspension) occurred in two discrete steps: an early fast change followed by a slow change. Changes in the filament forming ability of myosin also occurred in this early stage of heat treatment. A further study has been conducted by using myosins from skjpjack tuna, carp, rainbow trout, and squid. The inactivation of actomyosin ATP ade activity and the increase in turbidity of myosin suspensions also proceed in two steps upon heat treatment of all the myosins used. The order in the thermal stability of myosin filaments estimated from the rate of the fast inactivation of actomyosin ATP ase activity was rabbit> skipjack tuna>carp> rainbow trout>squid.This order was identical with that of the thermal stability previously estimated form the Ca-ATPase activity of myosin or actomyosin. The presence of actin or sucrose during the heat treatment effectively prevented any thermal denaturatin of myosin from occurring.