  We discuss Hubble Space Telescope optical images and spectra of NGC 6251, a
giant E2 galaxy and powerful radio source at a distance of 106 Mpc (for H_0 =
70 km/s/Mpc). The galaxy is known to host a very well defined dust disk (O'Neil
et al. 1994); the exceptional resolution of our V and I images allows a
detailed study of the disk structure. Furthermore, narrow band images centered
on the Halpha+[NII] emission lines, reveal the presence of ionized gas in the
inner 0.3 arcsec of the disk. We used the HST/Faint Object Spectrograph with
the 0.09 arcsec aperture to study the velocity structure of the disk. Dynamical
models were constructed for two extreme (in terms of central concentration)
analytical representations of the stellar surface brightness profile, from
which the mass density and corresponding rotational velocity are derived
assuming a constant mass-to-light ratio (M/L)_V ~ 8.5 M_solar/L_solar. For both
representations of the stellar component, the models show that the gas is in
Keplerian motion around a central mass ~ 4 - 8 X 10^8 solar masses, and that
the contribution of radial flows to the velocity field is negligible.
