The effect of inorganic polyphosphates, such as sodium pyrophosphate, sodium tripolyphosphate, and sodium POLYPHOSPHATE (Food Additive) on thermal denaturation of carp myofibrils olution (in 0.50 M KCI, pH 7.0) was examined. As the concentration of inorganic polyphosphate increased in the myofibrils solution, thermal inactivation rate (kD) of myofibrillar Ca-ATPase was insignificantly increased. However, to the myofibrils solution containing various concentrations of inorganic polyphosphate, KCI was further added to make its ionic strength fixed. Then, the kD of these myofibrils was altered to be almost identical. When sodium pyrophosphate and MgCl2 were added to the myofibrils solution, thermal inactivation rate of myofibrillar Ca-ATPase greatly increased. In addition, reaction mode of thermal inactivation of myofibrillar Ca-ATPase changed from a single first order process to biphasic first order behavior. It was also found that these changes were recovered upon addition of rabbit F-actin. These results strongly suggested that F-actin portion in the myofibrils was denatured more faster than its myosin portion during the treatment of myofibrils with inorganic pyrophosphate plus MgCl2 at high ionic strength.