Analysis of data recorded at an array of three-component broadband seismograph stations deployed on northern Vancouver Island and the adjacent British Columbia mainland, at the northern end of the Cascadia subduction zone, provides the first constraints on the S wave velocity structure of this region and permits us to define the northern limit of the subducted Juan de Fuca plate system. During a 2-year period, more than 80 teleseisms were recorded at our five stations. The method of receiver function analysis was used to constrain the S velocity structure to upper mantle depths. Beneath the northern three stations, a relatively simple continental crust is interpreted with a well-defined Moho near 37--39 km depth. An upper crustal S velocity discontinuity at these stations is interpreted as the top of the high-velocity rocks of the Wrangellia terrane. In contrast, more complicated structure dominated by pronounced low-velocity zones dipping to the NE are interpreted beneath our southern two stations. The shallower low-velocity zone is 6--8 km thick, has an S velocity contrast of 0.6--1.1 km/s, and lies within the continental crust. This feature is similar to a pronounced low-velocity layer (the E zone) imaged beneath southern Vancouver Island. The deeper low-velocity zone is interpreted as the subducted oceanic crust. We interpret the pronounced change in S velocity structure that we observe as the northern limit of the subducted oceanic plate beneath Vancouver Island. This change coincides with significant changes in topography, heat flow, gravity, and geochemistry. ¿ 1998 American Geophysical Union