<p>Three types of sarcoplasmic proteases, heat-stable alkaline protease “HAP”, thrombin, and low molecular types of cathepsin L, were identified from Alaska pollock muscle and six grades of commercially supplied Alaska pollock surimi. These proteases showed an ability to degrade myosin heavy chain under the condition of the modori-phenomenon. “HAP” and thrombin were removed by repeated homogenization and centrifugation of Alaska pollock muscle, whereas some cathepsin L activity remained in the four-times washed myofibrillar proteins. Activity of myofibril-bound serine protease (MBSP) in Alaska pollock myofibrillar proteins was at an insufficient level to cause the modori-phenomenon. In RAO grade of commercially supplied Alaska pollock frozen surimi containing higher and comparable total activity of cathepsin L and other proteases, rapid degradation of myosin heavy chain, which was similar to that in Alaska pollock muscle, was observed. However, in SA, FA, AA, A, and RB grades of surimi, the rate of myosin heavy chain degradation under conditions of the modori-phenomenon was not exactly consistent with total activities of “HAP”, thrombin, and cathepsin L. These results suggested that not only cathepsin L but also “HAP” and thrombin participate in the modori-phenomenon with different levels between each grade of surimi.</p>