We examined beta- (approximately 20 Hz) and gamma- (approximately 40 Hz) band activity in auditory cortices by means of magnetoencephalography (MEG) during passive listening to a regular musical beat with occasional omission of single tones. The beta activity decreased after each tone, followed by an increase, thus forming a periodic modulation synchronized with the stimulus. The beta decrease was absent after omissions. In contrast, gamma-band activity showed a peak after tone and omission, suggesting underlying endogenous anticipatory processes. We propose that auditory beta and gamma oscillations have different roles in musical beat encoding and auditory-motor interaction.