  We present the 21-cm rotation curve of the nearby galaxy M33 out to a
galactocentric distance of 16 kpc (13 disk scale-lengths). The rotation curve
keeps rising out to the last measured point and implies a dark halo mass larger
than 5 10^{10} solar masses. The stellar and gaseous disks provide virtually
equal contributions to the galaxy gravitational potential at large
galactocentric radii but no obvious correlation is found between the radial
distribution of dark matter and the distribution of stars or gas. Results of
the best fit to the mass distribution in M33 picture a dark halo which controls
the gravitational potential from 3 kpc outward, with a matter density which
decreases radially as R^{-1.3}. The density profile is consistent with the
theoretical predictions for structure formation in hierarchical clustering cold
dark matter models but mass concentrations are lower than those expected in the
standard cosmogony.
