  A well known problem in supersymmetric models is the presence of, lepton and
baryon number violating, dimension four operators. The traditional R parity
solution may not be suitable if one tries to incorporate supersymmetry into a
Planck scale theory. I propose a different solution in the context of realistic
string models. I show that realistic string models can give rise to custodial
nonabelian gauge symmetries under which only the leptons or quarks transform. I
explain how such symmetries arise in a class of free fermionic models that are
based on $Z_2\times Z_2$ orbifold with standard embedding. The custodial
symmetries forbid proton decay from dimension four operators while allowing R
parity violation.
