  We analyze nucleon decay modes in a no-scale supersymmetric flipped SU(5) GUT
model, and contrast them with the predictions for proton decays via dimension-6
operators in a standard unflipped supersymmetric SU(5) GUT model. We find that
these GUT models make very different predictions for the ratios $\Gamma(p \to
\pi^0 \mu^+)/\Gamma(p \to \pi^0 e^+)$, $\Gamma(p \to \pi^+ \bar{\nu})/\Gamma(p
\to \pi^0 e^+)$, $\Gamma(p \to K^0 e^+)/\Gamma(p \to \pi^0 e^+)$ and $\Gamma(p
\to K^0 \mu^+)/\Gamma(p \to \pi^0 \mu^+)$, and that predictions for the ratios
$\Gamma(p \to \pi^0 \mu^+)/\Gamma(p \to \pi^0 e^+)$ and $\Gamma(p \to \pi^+
\bar{\nu})/\Gamma(p \to \pi^0 e^+)$ also differ in variants of the flipped
SU(5) model with normal- or inverse-ordered light neutrino masses. Upcoming
large neutrino experiments may have interesting opportunities to explore both
GUT and flavour physics in proton and neutron decays.
