Magnetotelluric data acquired over the subducting Cocos plate in southern Mexico image the top of the oceanic plate for at least 150 km inland from the coast. Although fluid expulsion occurs in the accretionary prism, enough fluid appears to remain at the top of the subducting plate, because of sealing of pores and fractures on the underside of the continental plate, to produce the conductivity contrast necessary for electrical mapping. The results are supported by those of previous gravity and seismic refraction surveys that suggest the presence of more porous material on top of a denser, subducting oceanic crust. Earthquake epicenters also confirm the location of the top of the plate. Magnetotelluric data could, therefore, be used to map the Cocos plate along the entire Middle America Trench, where it is apparently broken into separate segments that subduct at different rates and dip at different angles.