  In the context of the minimal supersymmetric standard model (MSSM), we
discuss the introduction of exotic matter below the string scale $M_X$ in order
to achieve gauge unification at $M_X$ (a constraint of a large class of string
models). The possible types of exotic matter that can realise this are
investigated and its effect on the top quark mass $m_t$ is presented. The
implementation of a theory of fermion masses which utilises the exotic matter
is briefly discussed.
