1. Unit recordings have been made from the central ends of filaments of the masseter nerve in lightly anaesthetized cats. Evidence is presented to show that fusimotor activity may be distinguished from alpha motor activity. 2. During reflex cyclic movements induced by intra-oral stimulation, two distinct patterns of fusimotor firing emerged. One type of unit increased firing at the beginning and sustained this with little modulation throughout the movements. The other type was strongly modulated approximately in parallel with the alpha motor activity. 3. By comparison with records of jaw elevator spindle afferents under similar conditions, it was deduced that the sustained type of action was due to dynamic fusimotor neurones while the modulated type was due to static fusimotor neurones. 4. The patterns of fusimotor activity seen in these rhythmic movements under light anaesthesia agree well with the patterns deduced from spindle recordings in the conscious cat during mastication. 5. The results emphasize the importance of looking beyond a simple hypothesis of 'alpha-gamma co-activation' to explain fusimotor function. It is proposed that tonic dynamic fusimotor activity is set at the beginning of a movement to determine the incremental sensitivity of primary endings to stretch. The static fusimotor fibres are activated principally during shortening to help keep both primary and secondary endings active.