  We discuss the problem of the small ratio of muon mass to tau mass in a class
of seesaw models where maximal atmospheric neutrino mixing is enforced through
a $\mu$--$\tau$ interchange symmetry. We introduce into those models an
additional symmetry $T$ such that $m_\mu = 0$ in the case of exact $T$
invariance. The symmetry $T$ may be softly broken in the Higgs potential, and
one thus achieves $m_\mu \ll m_\tau$ in a technically natural way. We speculate
on a wider applicability of this mechanism.
