1) Every skeletal muscle contains three types of muscle fiber according to the content of sarcosome (mitochondria and fat body) in sarcoplasm which is the site of metabolism in the muscle fiber. In this study, these three types of fiber are classified as follows; sarcosome rich (red or dark), sarcosome poor (pale or light) and intermediate fiber. In general, sarcosome rich (red) fiber is thinner than the others, but that contained in functionally tonic proximal muscles of the extremity is thicker than the pale. The histological and histochemical differences among these three types of fibers are markedly seen in the lower animals than in the higher ones. 2) Extrinsic ocular muscles consist of three types of fiber with characteristic distribution. In the peripheral part of the muscle they are crowded with thin red fibers, but in the axial part the three types of fiber intermingle. 3) Red fibers in muscles with vigorous contractile power and also which take part in postural function have higher sarcosome density than the others. But muscles taking part in isotonic contraction and plastic tonus contain numerous intermediate fibers. Extremity muscles with phasic function have numerous sarcosome poor (pale) and a few intermediate and red fibers. Considering these findings, it seems that the three functions of the muscle are taken partial charge by the three types of fiber. That is, contractile tonus to sarcosome rich (red), phasic contraction to sarcosome poor (pale) and plastic tonus to intermediate fiber. 4) Each muscle fiber of these three types differs in their microscopic picture of contraction by indirect electrical stimulation to the muscle. Contraction knots are seen only in the red fibers and tetanizing picture in the pale ones. 5) Denérvated muscle atrophy appears more early in the pale fibers than in the red.