  A distinctive feature of string unification is the possibility of unification
by a non-simply-laced group. This occurs most naturally in four dimensional
type~II string models where the gauge symmetry is realized by Kac-Moody
algebras at different levels. We investigate the running coupling constants and
the one-loop thresholds for such general models. As a specific case, we examine
a $\rm SU(3)\times U(1)\times U(1)$ model and find that the threshold
corrections lead to a small $6\%$ increase in the unification scale.
