  We present a study of the decay of metastable states of a scalar field via
thermal activation, in the presence of a finite density of fermions. The
process we consider is the nucleation of ``{\it droplets}'' of true vacuum
inside the false one. We analyze a one-dimensional system of interacting bosons
and fermions, considering the latter at finite temperature and with a given
chemical potential. As a consequence of a non-equilibrium formalism previously
developed, we obtain time-dependent decay rates.
