M33: A galaxy with no supermassive black hole

Galaxies that contain bulges appear to contain central black holes whose ma sses correlate with the velocity dispersion of the bulge. We show that no c orresponding relationship applies in the pure disk galaxy M33. Three-integr al dynamical models fit Hubble Space Telescope WFPC2 photometry and Space T elescope Imaging Spectrograph spectroscopy best if the central black hole m ass is zero. The upper limit is 1500 M.. This is significantly below the ma ss expected from the velocity dispersion of the nucleus and far below any m ass predicted from the disk kinematics. Our results suggest that supermassi ve black holes are associated only with galaxy bulges and not with their di sks.