Electron microscopy, together with quantitation using a tracing device linked to a digital computer, reveals that the red and white muscle fibers of Salmo gairdneri differ in diameter, organization of myofibrils, dimensions of myofilaments, volumes and surface areas of T system and sarcoplasmic reticulum, morphology of mitochondria, and content of mitochondria, lipid, and glycogen. Biochemical studies show that the ATPase activity of white fibers is three times that of the red fibers. Actomyosin content of red fibers is higher than that of the white fibers. The functional significance of these differences between two fiber types is discussed.

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