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Muscle fiber distribution has been studied in the profound and the superficial parts of M. Biceps femoris of castrated males and females of two breeds (Duroc, Landrace) of pig. The results show that fiber types in the M. biceps femoris of pig can be conveniently classified into three categories on the basis of histochemical reaction; those are type I-red fiber with an alkali-labile and acid-stable myosin ATPase and a high succinic dehydrogenase (SDH) activity, type II-white fiber with an alkali-stable and acid-labile myosin ATPase and a low SDH activity, and type II-red fiber with an alkali-stable myosin ATPase and a high SDH activity. Type II-red fiber was moreover divided into two groups, fiber with an acid-labile myosin ATPase and fiber with an acid-stable myosin ATPase. The latter was only about 3% of all type II-red fiber. The values of x2 tes showing the difference of fiber type distributing between the profound and the superficial part were significant (p < 0.01) in the six pigs of seven. There are more abundant type I-red muscle fibers in the profound part, reciprocally more abundant type II-white muscle fibers in the superfical part than each another part. Type II-white fiber had a larger diameter in the superficial part of castrated males, and in the profound part of females than each another part in both breeds. The diameter of type I-red fiber of the profound part was larger htan that of the superficial part in the Duroc castrated males.