  We present a new scenario for gauge coupling unification in flipped SU(5)
string models, which identifies the $M_{32}$ scale of SU(3) and SU(2)
unification with the empirical $M_{\rm LEP}\sim10^{15-16}$~GeV scale, and the
$M_{51}$ scale of SU(5) and U(1) unification with the theoretical $M_{\rm
string}\sim5\times10^{17}$~GeV string unification scale. The vacuum shift
necessary for the cancellation of the anomalous $\rm U_A(1)$ and an SU(4)
hidden sector with fractionally-charged particles, play a crucial role in the
dynamical determination of all intermediate mass scales in this scenario.
