The magnetotelluric (MT) method is an electromagnetic sounding technique used in geophysical exploration that is based on the measurement of surface electric and magnetic fields. The presence of three-dimensional conducting heterogeneities near the surface of the Earth obscures conducting regions buried deeper in the Earth and leads to interpretational errors. In this paper a procedure referred to as ''wave number filtering'' is developed that dramatically reduces the undesirable effects of three-dimensional heterogeneities on the Zxy and Zxx components of the MT impedance tensor, where the x axis is arbitrarily determined by the line of the MT survey. The Zyx and Zyy components of the MT impedance tensor are improved to some extent by wave number filtering, but the amount of improvement depends on the location with respect to the anomalous region and does not, in general, result in a correct interpretation. The wave number filtering process is evaluated by comparing the filtered and the unfiltered apparent resistivities derived from the MT impedance tensor for a particular three-dimensional model consisting of a heterogeneous region on the surface of an otherwise homogeneous Earth. ¿ American Geophysical Union 1989