  We investigate the allowed ranges of masses for an unstable tau neutrino in
the context of SO(10) GUT-s. In light of the new nucleosynthesis results we
obtain that there is a narrow window for m_{\nu_\tau} where the LEP, neutrino
oscillation and nucleosynthesis data are compatible. This window, which depends
on the effective number of neutrinos contributing to nucleosynthesis, has
important cosmological consequences and will be tested by ongoing neutrino
oscillation and LEP II experiments.
