Dissections of the human sartorius, biceps femoris, semimembranosus and semitendinosus muscles were carried out to determine the length and mode of termination of the individual muscle fibers. In the human sartorius the muscle fibers are all shorter than the fasciculi to which they belong. The fibers are of 2 distinct types. The first type (Type I) [long dash]the commoner[long dash]has bluntly-rounded intrafascicular tips and the fiber is usually about 1/3 to 1/4 of the length of the fasciculus, so that the fascicular unit consists of 3 or 4 serially-placed muscle-fibers joined together by short fibrous bridges. The second type (Type II) has gradually tapering intrafascicular tips and the fibers are more variable in length (between 1.5 and 30 cm) and not arranged in series. In biceps femoris (both heads) and semimembranosus the muscle fibers are all as long as the fasciculus. The average length of the biceps femoris fiber, in the adult in the erect posture, is thus approximately 9 cm for the ischial head and 11 cm for the femoral; that of semimembranosus approximately 6 cm. The semitendinosus fibers pass the whole length of each fasciculus, but in this case the muscle is divided, by a tendinous intersection, into 2 serially-placed groups of fasciculi, proximal and distal. The average length of the proximal group of muscle fibers is 7 cm and of the distal group 11 cm. The intrafascicular endings of the sartorius muscle fibers and the tendinous endings of the sartorius, biceps femoris, semimembranosus and semitendinosus muscle fibers are described.