In Germany frozen battered and breaded fish portions like "fish fingers" are important fishery products which account to more than 25% of the entire production of the German fish processing industry. The raw material for the production of these items consists of two kinds of frozen blocks of fillets - single frozen (SF) and double frozen (DF) ones. The possibility to differentiate between SF and DF fillets was investigated by applying two types of differential scanning calorimeters, The transition temperatures as well as the transition enthalpies of both fillet types were not significantly different. Furthermore, no influence of the rigor state on the thermal stability of the fish proteins could be detected in frozen fillets. The differences in the transition temperatures measured by using the various calorimeters could be explained by the differences in scanning rates applied.