Effects of freezing and frozen storage on the specific heat capacity difference of native (not heat denatured) and heat denatured fillet (ΔdncP) of trout and herring have been studied for the first time. Single and double freezing of fillet induced a clear reduction of theΔdncP of up to 15 mJ g−1 K−1 at each freezing step. Frozen storage at −20 °C reduced theΔdncP of herring fillet but not of trout fillet. This is in agreement with the earlier detected instability of myosin transition temperature of herring and the stability of myosin transition temperature of trout [1]. The ΔdncP is suggested as a new parameter for the evaluation of protein stability at freezing and frozen storage of fish. It has been shown, that a reversible endothermic effect between 2 and 22 °C in herring fillet is accompanied by a decrease of the storage modulus in the chopped fillet, which characterises the flexibility of the proteins. This mechanical change and the endothermic signal are missed in the chopped trout fillet at heating from 2 to 22 °C. This cold, reversible relaxation of fish proteins can be understood as a requirement for subsequent protein denaturation at frozen storage.