To further understand cortical mechanisms for controlling bipedal (Bp) gait in humans, we recorded neuronal activity in supplementary motor area (SMA) of an unrestrained monkey walking either quadrupedally or bipedally on a treadmill. Single-unit recording was made from trunk/hind-limb regions of SMA. EMG activity was simultaneously recorded from up to 16 muscles in the trunk and four limbs. We then compared modulation patterns of neuronal activity with those of EMG activity during two modes of locomotion. We found that, during quadrupedal (Qp) locomotion, the activity of trunk and limb muscles during Qp locomotion displayed discrete bursts without tonic activity in each step cycle. For Bp locomotion, the EMG burst for all but forelimb muscles was drastically enhanced in both amplitude and duration. Further, majority of hindlimb muscles became to peak in the mid-stance phase and the trunk muscles disclosed substantial tonic activity, on which the enhanced burst was superimposed. Meanwhile, of 47 SMA neurons analysed, 40 displayed phasic and/or tonic components of activity during at least one of the two locomotor tasks. Of these task-related neurons, only 15 showed the same component of activity between Qp and Bp locomotion. The other disclosed various combinations of activity components for Bp locomotion, which were different from those for Qp locomotion. Strikingly, 20 neurons superimposed further component of activity upon that for Qp locomotion, thus the SMA activity was quite task-dependent. For Bp locomotion, in general, the tonic discharge frequency was higher than Qp locomotion and the phasic activity was also enhanced in both amplitude and duration. Importantly, during Bp locomotion, significant proportion of cells peaked in the mid-stance phase. Interestingly, 7 neurons did not show any task-related modulation, but did display burst activity closely related to the transition of locomotor pattern from Qp to Bp and vice versa, or turning movements of the head, which was superimposed upon on-going locomotor movements. These results suggest that, during locomotion, monkey SMA contributes to the control of stepping movements and maintaining truncal posture and could provide insight into the basis of frontal gait disorders in humans.