  Yukawa coupling constant unification together with the known fermion masses
is used to constrain SO(10) models. We consider the case of one (heavy)
generation, with the tree-level relation $m_b=m_\tau$, calculating the limits
on the intermediate symmetry breaking scales. This analysis extends previous
analyses which addressed only the simplest symmetry breaking schemes. In the
case where the low energy model is the standard model with one Higgs doublet,
there are very strong constraints due to the known limits on the top mass and
the tau neutrino mass. The two Higgs doublet case is less constrained. Finally
we address the role of a speculative constraint on the tau neutrino mass,
arising from the cosmological implications of anomalous $B+L$ violation in the
early universe.
