Human subjects were taught to generate constant forces with their lips by contracting across a pair of hooks which held the corners of the mouth fixed in place. The force was measured with a strain gauge attached to one of the hooks. The activity of single motor units was recorded during production of small to near maximal levels of force. The recruitment and firing patterns of each unit were observed, and in vivo contractile properties of each unit were estimated using spike-triggered averaging of the high-gain force record. Recruitment of units was observed at all levels of force and recruitment level varied from trial to trial for each unit. Interspike interval variability was high at all levels of force. The twitch tensions of the units were all less than 10 g; the contraction times ranged from 16 to 90 ms. It is hypothesized that the differences between lip and limb motor unit properties in this task reflect the mechanical, anatomic, and neurophysiologic differences between these systems.