  We investigate the mixing of heavy gauge singlet neutrinos in a mirror matter
model employing the seesaw mechanism. The parameter constraints that must be
satisfied to prevent the overproduction of mirror matter in the early universe
are deduced. We find that no fine tuning in the heavy neutral fermion sector is
required for this mirror matter model to satisfy cosmological constraints.
Baryogenesis scenarios are briefly discussed in the context of the mirror
model.
