A seismic broadside wide-angle experiment in Canada's southeastern Cordillera has revealed an anomalous and unexpected seismic arrival. Analysis of the data shows that the arrival is unlikely to be a crustal or shallow mantle phase or multiple or converted waves. The kinematic characteristics of this arrival dictate that it is a reflection from within, or from the base of, a low velocity zone (<7.8-8.0 km s-1) situated at a depth of 59 to 62 km that we identify as the top of the asthenosphere. This leads to the conclusion that the base of the lithosphere is at a depth of 50¿2 km in the region. The interpreted shallow asthenosphere bolsters earlier conclusions of a heat source at depth in the southeastern Canadian Cordillera. The interpretation also shows that stratification in the upper mantle is resolvable by current wide-angle seismic experiments.¿ 1997 American Geophysical Union