1. The discharge properties of individual motor units in different modes of voluntary contraction were studied with electromyographic techniques in the short toe extensor muscle of normal man. 2. The short toe extensor muscle consisted of type I and type II muscle fibres in about equal proportion. In some subjects there was type-grouping so that recordings with sufficient selectivity could easily be obtained. 3. Certain motor units could be driven continuously, attained regular firing intervals even at a firing rate of 10/sec, increased slowly in firing rate with increase in contraction strength, had maximum firing rate below 30/sec during sustained contraction but above 60/sec in twitch contraction. 4. Other motor units could not be dirven continuously, did not fire repeatedly at rates below 20/sec, increased rapidly in firing rate with increase in contraction strength and attained firing rates above 100/sec. 5. There were intermediate forms between continuously firing low frequency motor units and intermittently firing high frequency motor units. 6. In a prolonged contraction of constant strength only continuously firing motor units were active. 7. On rapid accelerations, however, both continuously and intermittently firing motor units were active and played about the same role. 8. This applied also to prolonged series of accelerations as in rhythmically alternating movements. 9. In twitch contractions selective activation of intermittently firing motor units occurred if the muscle was relaxed prior to the twitch and great effort was used to elicit the twitch and minimum duration of the twitch was intended. 10. It is suggested that continuously firing low frequency motor units have type I muscle fibres and intermittently firing high frequency units have type II muscle fibres and that the order of recruitment and the relative roles of the two motor unit types are adapted to the mode of contraction.