Supersymmetry and light quark masses in a realistic superstring model

We examine the light quark masses in a standard-like superstring model in the four-dimensional free fermionic formulation. We find that the supersymmetry constraints in the observable and hidden sectors eliminate all large contributions to mu and md and force them to be much smaller than the other quark masses. The requirement for an acceptable Higgs doublet spectrum results in mu<<md. In these models a realistic md can always be obtained whereas mu is at most 10-5 MeV. For particular choices of flat directions or vacua mu can be as small as 10-7 MeV but cannot vanish. ((orig.))