The muscle fiber conduction velocity is used as an indication of a localized muscular fatigue or some muscular diseases. The purpose of this study was non-invasively to estimate the conduction velocity of the human masseter muscle using surface electrode arrays. Informed consent was obtained from a group of 8 healthy subjects (24-27 yr) without functional disturbances of the jaws and with natural dentitions. Each subject was asked to perform a brief isometric contraction at 10, 20, 30, 40 and 50% of the maximum EMG amplitude and an endurance of 30 and 50% MVC in the intercuspal position. The myoelectric potential was derived from an adjacent pair of electrode wires and the velocity was estimated by the electrode distance divided by the time delay between two myoelectric signals. The time delay was calculated by the cross-correlation technique. The results obtained were as follows: 1. The muscle fiber conduction velocity slightly increased with contraction force. 2. The exponential reduction in velocity was shown during fatiguing contraction of 50% MVC. 3. The muscle fiber conduction velocity decreased with the time after fatiguing contraction. 4. It was suggested that the conduction velocity of the muscle membrane discharge was an important index of muscular fatigue.