  We suggest that photons with energies between 5 and 10 MeV, generated by the
($\nu,\nu'p\gamma$) and ($\nu,\nu'n\gamma$) reactions on $^{16}$O, constitute a
signal which allows a unique identification of supernova $\nu_\mu$ and
$\nu_\tau$ neutrinos in water \v{C}erenkov detectors. We calculate the yield of
such $\gamma$ events and estimate that a few hundred of them would be detected
in Superkamiokande for a supernova at 10 kpc distance.
