Expressions for the transmission cross section of an infinite slit excited by an H-polarized plane wave in a gyrotropic medium such as a plasma with a magnetostatic field parallel to the slit edges are obtained. For normal incidence these are similar to the isotropic form of the cross-section theorem and some numerical results are presented for a wide slit. They suggest that greatly enhanced power transmission through the slit occurs in a particular frequency range in the plasma.