1. Thermal stability of fish myosin has been studied by using differential scanning calorimetry (DSC) and circular dichroism (CD). 2. The temperature range of the sharp decrease in alpha-helical content agreed very closely with that of the endothermic peaks. 3. There was a high correlation between the enthalpy of denaturation (DELTAH) and the decreasing quantity in alpha-helicity (DELTAh). 4. The structure of fish myosins was much more unstable than that of rabbit. 5. The instability of fish myosins was reflected in its rod moiety.