Postnatal development and differentiation of the masseter muscles consisting only of fast-twitch oxidative (FO) fibers in the adult Japanese field vole Microtus montebelli were studied using his-tochemical and electron microscopic techniques. The masseter muscles were composed of myotubes and muscle fibers at day 0 (birth day). Most muscle cells showed the strong reaction for myosin ATPase after both alkaline and acid preincubations. For NADH-dehydrogenase (NADH-DH), small granular diformazan deposits were recognized in the sarcoplasm. Afterwards, the masseter muscles consisted of myofibers and satellite cells at day 5. For myosin ATPase, weakly-reactive fibers after acid preincubation (fast-twitch fibers) increased in number. For NADH-DH, granular diformazan deposits in all the myofibers increased in size. Since all the myofibers had numerous sarcoplasmic reticula, and they reacted strongly after alkaline preincubation and weakly after acid preincubation for myosin ATPase at day 10 when the young start to take solid food, it seems that the masseter muscles become contractive fast. At day 15 (before weaning), all the myofibers showed the adult-like strong reaction for NADH-DH and had numerous well-developed mitochondria, thus they acquired the ability of the fast and sustained contraction. It is accordingly considered that the masseter muscles of the vole mature in a short time after birth because of adaptation for herbivorous food habit.