  It is shown that in realistic SUSY GUT models of quark and lepton masses both
the proton decay rate and branching ratios differ in general from those
predicted in the minimal $SU(5)$ supersymmetric model. The observation of
proton decay, and in particular the branching ratio $B[(p \rightarrow \pi^+
\overline{\nu})/(p \rightarrow K^+ \overline{\nu})]$, would thus allow decisive
tests of these fermion mass schemes. It is shown that the charged lepton decay
modes $p \rightarrow K^0 \mu^+, p \rightarrow K^0 e^+$ arising through gluino
dressing diagrams are significant and comparable to the neutrino modes in large
tan$\beta$ models. Moreover, it is found that in certain classes of models the
Higgsino-mediated proton decay amplitudes are proportional to a model-dependent
group-theoretical factor which in some cases can be quite small. The most
interesting such class consists of $SO(10)$ models in which the dominant
flavor-symmetric contribution to the up-quark mass matrix comes from an
effective operator of the form ${\bf 16}_i{\bf 16}_j {\bf 10}_H {\bf 45}_H$,
where $\langle {\bf 45}_H \rangle$ points approximately in the $I_{3R}$
direction. This class includes a recent model of quark and lepton masses
proposed by the authors.
