  We have obtained long-slit observations of the circumnuclear region of M87 at
three different locations, with a spatial sampling of 0.028" using the Faint
Object Camera f/48 spectrograph on board HST. These data allow us to determine
the rotation curve of the inner ~1" of the ionized gas disk in [OII]3727 to a
distance as close as 0.07" (~5pc) to the dynamic center, thereby significantly
improving on both the spatial resolution and coverage of previous FOS
observations.
  We have modeled the kinematics of the gas under the assumption of the
existence of both a central black hole and an extended central mass
distribution, taking into account the effects of the instrumental PSF, the
intrinsic luminosity distribution of the line, and the finite size of the slit.
  We find that the central mass must be concentrated within a sphere whose
maximum radius is 0.05" (~3.5pc) and show that both the observed rotation curve
and line profiles are consistent with a thin--disk in keplerian motion. We
conclude that the most likely explanation for the observed motions is the
presence of a supermassive black hole and derive a value of M_{BH} =
(3.2+/-0.9) 10^9 M_{sun} for its mass.
