First arrival times from regional distances (200--1200 km) in the western United States extracted from the International Seismological Centre (ISC) data set were inverted in a tomographic study to map the laterally varying Pn velocity structure of the uppermost mantle (i.e., the mantle lid) and to estimate the crustal static delay at each seismograph station. Synthetic data were used to evaluate resolution. Results correlate well with major tectonic features. We find low uppermost mantle velocities (V7.9 km/s). These low apparent Pn velocities are primarily due to the effects of isostasy, hotspot volcanism, and crustal extension. We interpret the low apparent Pn velocities beneath the Sierra Nevada (7.6--7.7) km/s) to be the result of time delays introduced by ray paths tunneling through the deep crustal root. The Yellowstone hotspot, migrating along the Snake River Plain, has left low velocity (<7.6--7.8 km/s), hot upper mantle in its wake. Lower Pn velocities (-7.8 km/s) beneath the extensional Basin and Range are also presumably due to hotter uppermost mantle. Based on station static delays for the Basin and Range, we infer that Moho depths there do not vary significantly from 30 km. Normal continental Pn velocities (-8.0 km/s) as well as thicker crust (40--50 km) underlying the Rocky Mountains, Wyoming Basin Province, Colorado Plateau, and the northern Columbia Plateau indicate the presence of relatively cool uppermost mantle beneath these regions. ¿American Geophysical Union 1991