  We analyze the general structure of the fermion mass matrices in effective
superstrings. They are generically given at low energy by non-trivial functions
of the gauge singlet moduli fields. Interesting structures appear in particular
if they are homogeneous functions of zero degree in the moduli. In this case we
find Yukawa matrices very similar to the ones obtained by imposing a $U(1)$
family symmetry to reproduce the observed hierarchy of masses and mixing
angles. The role of the $U(1)$ symmetry is played here by the modular symmetry.
Explicit orbifold examples are given where realistic quark mass matrices can be
obtained. Finally, a complete scenario is proposed which generates the observed
hierarchies in a dynamical way.
