Two types of thermal scanning rigidity monitors (TSRM) were developed which are nondestructive and capable of monitoring rigidity or viscosity changes during heating of proteins over a wide range of concentrations. Thermal transitions which occur during gelation of croaker actomyosin were studied by these TSRM devices and the Brookfield viscometer during constant rate heating (1°C/min). Gelation of actomyosin occurred only at protein concentrations above 5.5% under these conditions. In plots of rigidity versus temperature, three transitions were observed during gelation, occurring near 38°C 46°C and 60°C. At lower concentrations, only one peak was observed, occurring near 36°C. A relationship between the 36–38°C transition in rheological properties and the high temperature “setting” phenomenon of fish proteins is postulated.