  The nature of R-parity violating interactions in two classes of string
inspired supersymmetric grand unified theories (SISUSY GUT), based on the gauge
groups $SO(10)$ (and its subgroup $SU(2)_L\times SU(2)_R\times SU(4)_c\equiv
G_{224}$) as well as $[SU(3)]^3$, are discussed and their strengths are related
to the ratio of symmetry breaking scales present in the model. We first argue
that for the R-parity violating couplings $\lambda_{R\!\!\!/}$ to be suppressed
to the desired level, the $B-L$ local symmetry must break at an intermediate
scale $M_{B-L}$ since $\lambda_{R\!\!\!/} =M_{B-L}/M_{GUT}$. We then construct
scenarios where such intermediate scales arise being consistent with gauge
coupling unification in a two-loop renormalization group study. In the
resulting $SO(10)$ models, higher-dimensional-operator-induced R-parity
violating couplings are potentially large (except in one case), and are
therefore inconsistent with limits on the proton life time unless the couplings
associated with the higher dimensional terms are fine tuned to very small
values. However, the $[SU(3)]^3$ and $G_{224}$ models can be consistent if a
certain class of quark-lepton coupling in the superpotential is forbidden by a
discrete summetry (unrelated to R-parity). An interesting prediction of these
models is neutron-anti-neutron oscillation with observable strength.
