We investigate S velocity structure beneath the Southwestern U.S. and Mexico using upper mantle turning S waves and surface waves. The data are tangential component records from Baja California, Mexican and Central American events recorded at broadband station ANMO (Albuquerque, New Mexico). Study geometry allows investigation of the variation of S structure with source-receiver azimuth for sources from Baja to Yucatan. The narrow backazimuthal sector studied comprises a transition from oceanic to tectonic to more 'continent-like' structure. We document this transition in the lithosphere and upper mantle to roughly 450 km depth. ¿ 1997 American Geophysical Union