The lithospheric structure beneath the Colorado Plateau and Great Basin of the western United States is investigated using Pn and surface wave data from the Colorado Plateau-Great Basin broadband seismic experiment. Average Pn velocities of 8.0 km/s in the Colorado Plateau and 7.8 km/s in the eastern Great Basin are found. Teleseismic Rayleigh waves are also measured, and eastern Great Basin phase velocities are found to be 3--4% higher than Colorado Plateau phase velocities in the 20--35 s range, but indistinguishable at periods greater than 40 s. The apparent Pn/surface wave discrepancy is resolved with models that include a thicker crust and faster mantle lid for the Colorado Plateau than the eastern Great Basin. Models for both regions require a mantle low velocity zone. The slow mantle deep beneath the Colorado Plateau may suggest a thermal origin of Plateau uplift, which has not yet penetrated the mantle lid. ¿ 2001 American Geophysical Union