The spike-triggered averaging technique was used to determine the time course and extent of fatigue of single motor unit twitches in the human masseter. This is the first report of a fatigue test having been applied to masseter motor units in either animals or humans. The human masseter was found to be comprised predominantly of fast-twitch motor units with a broad spectrum of fatigability. Very few physiological type S units were found, despite histochemical evidence for a substantial population of type I fibers in the masseter. In addition, there was no significant correlation between fatigability and either twitch amplitude or contractile speed in the motor units studied. The latter observations are consistent with the unusual histological features of the masseter. Comparison with other human fatigue data suggests that the extent of fatigue in the present population of masseter motor units after approximately 3,000 activations is similar to that reported for populations of units in first dorsal interosseous and medial gastrocnemius.