The nature and distribution of 2 types of fibers in the rat diaphragm, with respect to its dorsal (vertebral), lateral (costal), and ventral (sternal) regions was studied. Histo-chemical and quantitative studies on the fat and glycogen contents and enzymic activity (lipase and succinic dehydrcgenase) were made in these regions. It is concluded that the narrow red type of fiber is well adapted for aerobic metabolism, involving mainly the oxidation of fat for energy, and the broad white variety for anaerobic metabolism where glycogen is the chief fuel. The significance of the regional differences in the 2 types of fibers in the physiology of the diaphragm is discussed.