  The linear hydrodynamic stability of proto-neutron stars (PNSs) is considered
taking into account dissipative processes such as neutrino transport and
viscosity. We obtain the general instability criteria which differ essentially
from the well-known Ledoux criterion used in previous studies. We apply the
criteria to evolutive models of PNSs that, in general, can be subject to the
various known regimes such as neutron fingers and convective instabilities. Our
results indicate that the fingers instability arises in a more extended region
of the stellar volume and lasts a longer time than expected.
