Simultaneous electromyographic records were made from three thenar and the three hypothenar muscles of twenty-five mormal subjects, to determine the function of these muscles in various postures and grips of the hand. Extremely fine, indwelling wire electrodes were used. It was found that not all thenar muscles were active in all thumb positions, but that all hypothenar muscles were active in three basic postures of the little finger. Two somewhat different patterns activity occurred when the thumb was first softly and then firmly opposed to each of the fingers in a sequence that began at the index and ended at the little finger. The flexor pollicis brevis was dominant in firm grip, particularly in grip between the thumb and two radial fingers, but a large degree of abduction of the thumb might possibly have been a limiting factor in the activity of this muscle. The two opponents muscles seemed to act as a unit in opposition of the thumb to both the ring and little fingers. Certain activity observed in some of the six muscles and inexplicable on a morphological basis probably served to provided stability.