To elucidate the relation between the caliber of myelinated nerve fibers (MNFs) and motor unit size (i.e., number of muscle fibers per motor unit), motor unit size was measured for the mouse m. stylohyoideus, innervated predominantly by extra large MNFs, and compared with that of the mouse m. stapedius, innervated predominantly by large MNFs. All muscle fibers and MNFs were counted morphologically, and mean motor unit size was calculated for each muscle. The results showod that the average of 12 mean motor unit sizes for the m. stylohyoideus (35.8±8.2), innervated predominantly by extra large MNFs, was greater than that for them. stapedius (6.5±1.3), innervated predominantly by large MNFs.