An unusually high-amplitude, midcrustal reflector was discovered by a Lithoprobe seismic reflection survey crossing a predominantly plutonic region in southwestern British Columbia, Canada. The ''bright spot'' was observed adjacent to Mount Cayley, a large Quaternary volcanic center associated with the Cascadia subduction zone magmatic arc. Characterization of the bright spot may advance our understanding of either the midcrustal structure of magmatic supply systems or the presence and storage of saline fluids in the midcrust. The location and three-dimensional geometry of the reflector were determined using travel time inversion. The resulting suite of models consistently describe a northwest dipping reflector that lies 12.5 to 13 km beneath the Mount Cayley volcanic complex. The areal extent of the reflective surface is 3 km by 1 km and the structure is less than 1.6 km thick. The transition between the body and the country rock must be rapid (<200 m) and exhibit a large impedance contrast in order to generate reflections of such high frequencies (30 Hz) and relative amplitudes (8.0 dB). Our preferred interpretation for the reflector is a fossil sill complex associated with the volcanic development of Mount Cayley. However, the data do not rule out the possibility that the reflective structure represents melt lenses or saline fluid produced by the dewatering of the subducted slab. The Mount Cayley bright spot lies just below the brittle upper crust, as do the midcrustal reflectors detected beneath Japanese subduction arc volcanoes. This correlation suggests that thermal and rheological properties just below the brittle/ductile crustal transition may be favorable for reflector formation. ¿ American Geophysical Union 1996