  The stability properties of newly born neutron stars, or proto--neutron
stars, are considered. We take into account dissipative processes, such as
neutrino transport and viscosity, in the presence of a magnetic field. In order
to find the regions of the star subject to different sorts of instability, we
derive the general instability criteria and apply it to evolutionary models of
PNSs. The influence of the magnetic field on instabilities is analyzed and the
critical magnetic field stabilizing the star is obtained. In the light of our
results, we estimate of the maximum poloidal magnetic field that might be
present in young pulsars or magnetars.
