The activity of buccinator, orbicularis oris superior and inferior, quadratus and triangularis muscles was recorded by surface and needle electrodes during spontaneous unilateral mastication of small or large boluses. Their activity was compared with that of the masticatory muscles masseter and digastric, and with the vertical movement of the jaw. The facial muscles were active during mastication, displaying both tonic and phasic activity. Although the cyclic activity was broadly linked to lowering of the jaw, there was no strict time correspondence between the activity of the various facial muscles themselves, or between the activity of the facial muscles and the digastric. The timing and amplitude of facial muscle activity were affected by the duration of the masticatory cycle, the side on which chewing took place, the size of the bolus, and whether or not lip to lip contact was made.