  A generic feature of string-derived models is the appearance of an anomalous
Abelian U(1)_A symmetry which, among other properties, constrains the Yukawa
couplings and distinguishes the three families from each other. In this paper,
we discuss in a model-independent way the general constraints imposed by such a
U(1)_A symmetry on fermion masses, R-violating couplings and proton-decay
operators in a generic flipped SU(5) x U(1)' model. We construct all possible
viable fermion mass textures and give various examples of effective low-energy
models which are distinguished from each other by their different predictions
for B-, L- and R-violating effects. We pay particular attention to predictions
for neutrino masses, in the light of the recent Super-Kamiokande data.
