The heat stability of the proteins present in cod, siganus and tilapia fish muscle were compared using differential scanning calorimetry and by measuring the solubility of the proteins in weak and strong salt solutions following heat treatment at selected temperatures from 25" to 100°C for 1 hr. It was found that the stability of both the collagens of the connective tissue and the myosins of the myofibrillar proteins varied between species, the more stable proteins being those from the fish found in the waters of higher ambient temperatures. The stability of the sarcoplasmic proteins also exhibited some species dependence but the actins present in the three species were all of similar thermal stability. The thermograms obtained by scanning calorimetry indicated that during frozen storage some changes in the nature of the myosin molecule occurred. It is suggested that scanning calorimetry may be a rapid and simple means of following the so-called cold store denaturation of frozen fish but more work is required to confirm this.