  Core-collapse supernovae emit of order $10^{58}$ neutrinos and antineutrinos
of all flavors over several seconds, with average energies of 10--25 MeV. In
the Sudbury Neutrino Observatory (SNO), a future Galactic supernova at a
distance of 10 kpc would cause several hundred events. The $\nu_\mu$ and
$\nu_\tau$ neutrinos and antineutrinos are of particular interest, as a test of
the supernova mechanism. In addition, it is possible to measure or limit their
masses by their delay (determined from neutral-current events) relative to the
$\bar{\nu}_e$ neutrinos (determined from charged-current events). Numerical
results are presented for such a future supernova as seen in SNO. Under
reasonable assumptions, and in the presence of the expected counting
statistics, a $\nu_\mu$ or $\nu_\tau$ mass down to about 30 eV can be simply
and robustly determined. This seems to be the best technique for direct
measurement of these masses.
