  Common to many classes of three family SU(3)_C x SU(2)_L x U(1)_Y string
models are an additional anomalous U(1)_A and numerous, often fractionally
charged, exotic particles beyond the minimal supersymmetric standard model
(MSSM). I present one string model with both of these properties. Using this
model as an example, I show how sometimes (near) string scale vacuum
expectation values that are both D- and F-flat (to all orders in the
superpotential) can simultaneously break the anomalous U(1)_A and give heavy
mass to all phenomenologically dangerous exotic states, decoupling them from
the low energy spectrum. For this model, some anomaly cancelling directions
actually decouple all MSSM exotics, producing the MSSM as the effective field
theory just below the string scale. This model realizes Witten's conjecture of
possible equivalence between the string scale M_S and the MSSM unification
scale M_MSSM. Talk presented at CPT '98, Bloomington, Indiana, 6--8 November
1998
