Muscle fiber fine structure was quantitatively analyzed in 70 longitudinally and 65 transversely sectioned fibers from 10 control subjects without weakness. The average mitochondrial fraction of the fiber volume is close to 4 percent and the mean size of a mitochondrion is about 0.1 μm2. The sarcotubular surface area per unit fiber volume is close to 1.5 μm2/μm3 in transverse sections and 0.65 times this value in longitudinal sections. Only one-third of all fibers contain lipid droplets in the sectioned plane, and for all fibers the droplets account for approximately 0.12 percent of the fiber volume. Variations with the age and sex of the subjects and with different muscles were analyzed and the feasibility of typing human muscle fibers at the ultrastructural level was evaluated.