The distinctive cytological features of the red and white fibers of the pigeon pectoralis muscle have been studied with the electron microscope. In the red fibers, which are more numerous, numerous fat droplets occur between the myofibrils, as do large mitochondria, some of which attain giant size (4 × 1.3 μ). The white fibers, which are larger than the red, contain fewer and smaller mitochondria whose cristae are not so closely packed and the matrix not so dense as in the mitochondria of the red fibers. The white fibers contain large concentrations of glycogen granules but not fat droplets. Conspicuously large aggregations of mitochondria occur in the subsarcolemmal region as they do also in the immediate vicinity of the capillaries of both fibers, but those of the white fibers are markedly smaller. Some myofibrils of red fibers are closely associated with the capillaries, and it is suggested that their contractile activity effects muscular pumping of the blood in the capillaries to facilitate the intramuscular transport of oxygen and metabolites. The triads of the sarcoplasmic reticulum in the white fiber occur at the level of the Z-line. In the red fiber, however, they seem to be located at the A-I junction but may also be seen at the Z-line and at varying levels between the Z-line and the A-I junction. The Z-line in the red fibers is thicker than that in the white fibers. The M-band is prominent in both types of fiber. From the nature of the intracellular organization of the two types of fiber, it is suggested that the red fibers are "slow twitch" fibers and the white fibers "fast twitch" fibers. The former are engaged in sustained activity whereas the latter are active only for short periods of time.