Leg muscles of Japanese macaques (Macaca fuscata) were examined to characterize histochemical properties of myofiber types. Myofiber types were classified by differences in reactivity for myosin ATPase, NADH tetrazolium reductase (NADH-TR), and menadione-linked glycerol-3-phosphate dehydrogenase (3-GPD). The myofibers that reacted strongly for acid-stable myosin ATPase and were weakly reactive or unreactive for alkali-stable myosin ATPase were classified as slow-twitch/oxidative (SO) myofibers, which reacted strongly for NADH-TR and weakly to moderately for 3-GPD. The myofibers that were unreactive to moderately reactive for acid-stable myosin ATPase and strongly reactive for alkali-stable myosin ATPase were classified into fast-twitch/oxidative/glycolytic (FOG) myofibers with a moderate to strong activity for NADH-TR and 3-GPD and into fast-twitch/glycolytic (FG) myofibers with a weak NADH-TR activity and a strong 3-GPD activity.Slow-twitch myofibers with a strong activity for NADH-TR and 3-GPD were characterized as slow-twitch/oxidative/glycolytic (SOG) myofibers. The presence or absence of the SOG myofibers depended on the individuals.The soleus muscle had larger percentages of SO myofibers than of FOG and FG myofibers. The gastrocnemius and flexor digitorum superficialis muscles generally had larger percentages of FG myofibers than of FOG and SO myofibers including SOG myofibers.In mammals, myofibers of skeletal muscles are classified into three types (I, IIA, and IIB) by differences in reactivity for acid-stable myosin ATPase as shown by Brooke and Kaiser (3). Type I myofibers show a strong reaction for myosin ATPase after preincubation at pH 4.3-4.5.The myosin ATPase reaction was almost wholly inhibited at pH below pH 4.5 in type IIA myofibers and almost completely inhibited in type JIB myofibers below pH 4.3. The histochemical characteristics of type I myofibers are generally similar to those of the slow-twitch/oxidative (SO) myofibers on the basis of histochemical, biochemical, and physiological profiles as shown by Peter et al. (10). On the other hand, it has been shown that the histochemical characteristics of type IIA and type JIB myofibers do not always correspond to those of the fast-twitch/oxidative/glycolytic (FOG) and the fast-twitch/glycolytic (FG) myofibers, respectively (8,11).In prosimian primates, myofibers have been classified into FG, FOG, and SO myofibers by differences in reactivity for myosin ATPase and dehydrogenases (12).The myofibers have been classified into FG, FOG, and SO myofibers in cynomolgus monkeys (1) or types I, IIA, and JIB myofibers in rhesus monkeys (7) on the basis of reactivity for myosin ATPase and dehydrogenases.The purpose of the present study was to examine histochemical characteristics of myofibers in skeletal muscles of Japanese macaques (Macacafuscata) and to type their myofibers.Also, it was determined whether the myofiber types based on the classification system of Peter et al. (10) were interchangeable with those on the classification system of Brooke and K...