  We study the problem of baryon stability in M theory, starting from realistic
four-dimensional string models constructed using the free-fermion formulation
of the weakly-coupled heterotic string. Suitable variants of these models
manifest an enhanced custodial gauge symmetry that forbids to all orders the
appearance of dangerous dimension-five baryon-decay operators. We exhibit the
underlying geometric (bosonic) interpretation of these models, which have a
$Z_2 \times Z_2$ orbifold structure similar, but not identical, to the class of
Calabi-Yau threefold compactifications of M and F theory investigated by Voisin
and Borcea. A related generalization of their work may provide a solution to
the problem of proton stability in M theory.
