Magnetotelluric (MT) and geomagnetic depth sounding (GDS) data were recorded at 25 locations across Vancouver Island, Canada, over the subducting Juan de Fuca plate. The GDS data respond to a number of local conductive features at frequencies above 10 Hz while at longer periods, the in-phase and quadrature induction arrows axe colineax and are aligned approximately normal to the continental shelf with a maximum amplitude response neax the southwest coast. Interpretation of the MT data show the presence of a conducting zone at depths greater than 20 km beneath Vancouver Island and correlating with a strong seismic reflector. Extensive testing with two dimensional models shows the MT and GDS data axe consistent with a proposed geoelectrical structure containing a conducting layer extending from the Pacific Ocean, dipping beneath Vancouver Island, and in electrical contact with the mainland conductive region. The layer has a horizontal conductance of 200 S that may decrease to 100 S beneath the northeast side of Vancouver Island. Related geophysical studies suggest the conductive zone is above the actual tlmast surface of the Juan de Fuca plate. The conducting layer is most likely the result of cracks and pores filled with saline fluids which axe supplied by water subducted with the oceanic crust and by dehydration reactions. The presence of fluids has significant implications for thrust eaxthquakes and for metamorphic reactions that occur in subduction zones. The conductance of the upper mantle (between 90 and 390 km) is best modelled at approximately 1500 S. of the plate provides an excellent target for studying part of the western North American plate which is curthe electrical structure of a subduction zone. Kurtz et rently overriding the Juan de Fuca plate. Much of Vanal. [1986] presented a summary of the MT interpre-couver Island is part of the Wrangellia terrane [Jones et al., 1977], a thick assemblage of Paleozoic, Mesotation results including a highly conducting zone near the top of the subducting plate at the same depth as a zoic and Cenozoic volcanic, plutonic, sedimentary and strong seismic reflective zone [Green et al., 1986]. This metamorphic rocks [Muller, 1977]. Wrangellia was acpaper gives a more detailed interpretation and includes expanded discussions of the MT and GDS data.