  Lepton flavour violation has been proposed as a significant test of
supersymmetric unification. Here we show that such signals are also a generic
feature of supersymmetric string unified models in which there is no simple
unified gauge group. In realistic models of this kind which involve third
family Yukawa unification and large values of $\tan \beta$, there are generally
heavy right-handed (singlet) neutrinos of intermediate mass $M_{\nu}$, whose
couplings violate lepton flavour. To illustrate these effects we calculate the
rates for $\mu \to e \gamma$ and $\tau \to \mu gamma$ in the minimal
supersymmetric $SU(4)\times SU(2)\times SU(2)$ model. Including only the
minimum irreducible contributions, we find that both rates are enhanced
relative to similar models with low $\tan \beta$, with $\tau \to \mu gamma$
providing a decisive test of such models in the near future.
