  We systematically study the possibility for realizing realistic values of
lepton mass ratios and mixing angles by using only renormalizable Yukawa
couplings derived from heterotic $Z_6$-I orbifold. We assume one pair of up and
down sector Higgs fields. We consider both the Dirac neutrino mass scenario and
the seesaw scenario with degenerate right-handed majorana neutrino masses. It
is found that realistic values of the charged lepton mass ratios, $m_e/m_\tau$
and $m_\mu/m_\tau$, the neutrino mass squared difference ratio, $\Delta
m^2_{31}/\Delta m^2_{21}$, and the lepton mixing angles can be obtained in
certain cases.
