Coherent 20-35 Hz (beta) oscillations are a prominent feature of activity in primary motor cortex and muscles of monkeys and humans performing voluntary movements. We found that coherent beta oscillations are also present in the cerebellar nuclei (CN). Two monkeys were operantly conditioned to perform a wrist flexion/extension step-tracking task while we recorded neuronal activity or microstimulated in CN and recorded EMG activity from forelimb muscles. Coherent beta oscillations were found between discharges of some CN neurons and tonically active shoulder, elbow and wrist/finger flexion and extension muscles. Similarly, localized microstimulation pulses in CN evoked transient beta oscillations in widespread forelimb muscles. We conclude that coherent motor system beta oscillations are present in CN and that CN may be an important nodal point for the generation and/or propagation of beta oscillations throughout the motor system.