The object of this study is to estimate objectively the species difference and the changes in he physical properties of fish meat during the post-harvest storage and to search for the changes of the muscular matters underlying the property changes. Muscles of five fish species: skipjack, flyingfish, common horse mackerel, plaice, and channel rock fish were stored at 4°C for 14 days. Sample fish were taken out in turn and submitted to measurements of physical properties, namely penetration of a needle, firmness and cohesiveness by a General Foods type Texturometer, respectively. The measured values of each measurement item varied from species to species. During the storage, each of them shifted to the values reflecting the softening of the muscle usually observed. The rate of softening varied from species to species. The extractability of myofibrillar-, sarcoplasmic-, alkali-soluble-, and stroma-protein fractions of five fish muscles did not show any significant changes during storage. The SDS-polyacrylamide gel electrophoretic patterns of these extracted protein fractions and of the myofibrillar proteins extracted after M. H. STROMER showed no difference with respect to the storage time. The muscle samples were broken by rotating blades and sieved by 7 mesh, the percentage of the pieces remaining on the sieve decreased with the decrease of the freshness. The muscle samples were homogenized and the length of the myofibril fragments or the sarcomere number of each fragment was estimated under a microscope. The percentage of the small myofibrils varied by the fish species, and the degree of fragmentation increased with the storage period. These results denoted that changes in the physical properties, namely softening of meat, during the post harvest storage were affected more by the changes of the muscle tissue structures than by the changes of the component proteins.