The influence of straight dislocations on the second order nonlinear optical susceptibility &khgr;(2)of cubic noncentrosymmetric media has been modeled. The resulting degradation of &khgr;(2)has been evaluated for screw and edge dislocations in GaAs. It is found that single screw and edge dislocations reduce the &khgr;(2)of GaAs by a third and ∼60%, respectively. This implies that second order nonlinear optical effects are very sensitive to misfit dislocation generation in epitaxially grown noncentrosymmetric media and that dislocation‐free epilayers are required to realize the optimum &khgr;(2). ©1995 American Institute of Physics.