The I segment length has been measured and the number of thin filament periods per I segment has been counted in electron micrographs of Rhesus monkey and human leg muscle fibers. The number of I segment periods in the monkey fiber (60) and in the human fiber (66) multiplied by axial periodicity (385 Å) in thin filaments of living fibers (Huxley and BROWN, '67) plus 1,000 Å allowed for Z line width in living fibers, provided a formula for calculation of I segment lengths in living fibers. The calculated I segment lengths are 2.41 μ for the monkey and 2.64 μ for the human fiber. The ratio of calculated I segment length over an assumed constant thick filament length (1.6 μ) among vertebrates is in close agreement with the ratio of measured I segment length over measured thick filament length in micrographs of frog, rat, monkey and human leg muscle fibers. The calculated I segment length in the frog (1.95 μ) is compared with that of the human (2.64 μ) to show the relation of I segment length to the length-developed tension diagram and to the series elastic components of the skeletal muscle fiber.