The popliteus muscle was studied during both normal gait and a series of planned exercises. Its activity during the walking cycle was shown to begin shortly before heel-strike and to continue throughout three-quarters of the stance phase. During the exercises the muscle was active during internal rotation of the tibia on the femur, and during normal gait the phasic activity of the popliteus coincided with internal rotation of the tibia on the femur. It was concluded that the basic function of the popliteus is to help to bring about and maintain internal rotation of the tibia on the femur.