In the hummingbird, the supracoracoideus, the chief elevator muscle of the wing, though smaller than the depressor muscle pectoralis, is relatively better developed than in other birds. It is also unique in its cellular organization in that it consists of only the red (type 1) fibers like the pectoralis. Both muscles have conspicuously large aggregations of mitochondria which may be so numerous and densely packed in certain areas that the number of myofibrils in the area may be relatively insignificant, a feature hitherto unknown and unparalleled in vertebrate skeletal muscles. Of the two muscles the supracoracoideus appeared to contain larger aggregations of mitochondria, suggesting greater mitochondrial density in that muscle. This finds support in the biochemical observations of earlier investigators that, in hummingbirds, the supracoracoideus has a higher level of succinic dehydrogenase activity than the pectoralis. The ability of the hummingbird for hovering flight is attributed to the above unique features in its pectoral muscles.With the numerous mitochondria and fat droplets interspersed between myofibrils, some myofibrils tend to split and bifurcate. Similar splitting is also seen in regions where there is considerable proliferation of the sarcoplasmic reticulum. Numerous glycogen granules are present within the sarcoplasmic reticulum. Both muscles have copious blood supply through numerous capillaries. The disposition of the myofibrils in relation to the capillaries indicates that muscular pumping should be an effective mechanism for intramuscular transport of oxygen and metabolites.