The superficial red muscle of lingcod (Ophiodon elongatus) was shown to exhibit unique properties of post-mortem contraction and tension development. In comparison with white muscle, rigor contraction and isometric rigor tension in red muscle were about three times as great. The rate of contraction of the red muscle was dependent on temperature and also on the oxygen concentration in the surrounding atmosphere. The elastic modulus of the red muscle of trout and lingcod increased with increasing post-mortem time. Following the onset of rigor mortis a gradual increase in elasticity was observed. The maximum effects of contraction, tension and elasticity coincided with the onset of rigor mortis and each could therefore be used as a measure of this phenomenon. It was concluded from these experiments that stiffening of a fish with the onset of rigor mortis is not due to contraction or tension development of the muscles, but rather to their changing mechanical properties. A convenient measure of the changing mechanical properties in the muscle was the elastic modulus.Morphological differences between the very active, myoglobin rich, red muscle and the white muscle of lingcod were demonstrated by means of electron micrographs. The high glycogen content in the area of sarcoplasm of the red muscle, as indicated in electron micrographs, was confirmed by chemical analysis. Red muscle in rested fish was shown to contain from 1 to 3 times more glycogen than white muscle.