Threshold corrections and gauge symmetry in twisted superstring models.

Threshold corrections to the running of gauge couplings are calculated for superstring models with free complex world sheet fermions. For two N=1 SU(2) U(1) models, the threshold corrections lead to a small increase in the uni cation scale. Examples are given to illustrate how a given particle spectrum can be described by models with di erent boundary conditions on the internal fermions. We also discuss how complex twisted fermions can enhance the symmetry group of an N=4 SU(3) U(1) U(1) model to the gauge group SU(3) SU(2) U(1). It is then shown how a mixing angle analogous to the Weinberg angle depends on the boundary conditions of the internal fermions.