  We show that a recent proposal by Nambu to generate a hierarchy among Yukawa
couplings in the standard model may be easily implemented in superstring
models. In such models, two of the main ingredients of the Nambu proposal find
a natural explanation: minimising with respect to the Yukawas amounts to a
minimisation with respect to the underlying moduli fields and a constraint on
the Yukawas of the type of the Veltman condition may be attributed to the
relaxation process to a phenomenologically viable string vacuum.
