COCORP deep reflection data on the Colorado Plateau reveal complex reflections to about 16 s two-way travel time (50+ km), beyond which there is an abrupt decrease of reflectivity. This boundary is interpreted to represent the Moho, an inference consistent with the existing but limited refraction data sets when they are reconciled with the recent independent determination of an 8.10--8.15 km/s Pn velocity beneath the Colorado Plateau. This ~50 km depth to Moho and 8.10--8.15 km/s Pn velocity for the Colorado Plateau closely resembles that of the High Plains just east of the Rocky Mountains. The absence of continuous strong reflections at the Moho suggests that the Moho beneath the Colorado Plateau is a transition, not a velocity step function on the scale of the reflection experiment. The reflection boundary may represent the base of complex and discontinuous reflectors of the crust and crust/mantle transition, below which the mantle may be largely peridotite with a velocity of 8.1 km/s. These results strongly reinforce the suggestion that the Moho on deep reflection data in relatively stable continental regions is represented by an abrupt decrease in reflectivity, below which the mantle is relatively seismically transparent. ¿ American Geophysical Union 1989