The nature of masticatory movement and related muscle spindle afferent activity was studied on awake monkeys. The pressure acting on the molar surface of the tooth and masseter EMG may develop in two phases, i. e. an early jaw movement and a late biting. Out of 20 identified spindle afferents, 10 units in ten monkeys showed a rate increase during jaw-muscle closing and the remaining 10 units showed a decrease of the rate. Although there were two phases to the masseter contraction, no correlation was found between the two phases and the spindle afferent discharge pattern. It was concluded that during voluntary jaw closing the fusimotor activation occurred simultaneously with masseter muscle activation.