  We investigate the problem of the fermion mass hierarchy in supergravity
models with flat directions of the scalar potential associated with some gauge
singlet moduli fields. The low-energy Yukawa couplings are nontrivial
homogeneous functions of the moduli and a geometric constraint between them
plays, in a large class of models, a crucial role in generating hierarchies.
Explicit examples are given for no-scale type supergravity models. The Yukawa
couplings are dynamical variables at low energy, to be determined by a
minimization process which amounts to fixing ratios of the moduli fields. The
Minimal Supersymmetric Standard Model (MSSM) is studied and the constraints
needed on the parameters in order to have a top quark much heavier than the
other fermions are worked out. The bottom mass is explicitly computed and shown
to be compatible with the experimental data for a large region of the parameter
space.
