Seismic waves from nuclear explosions and from well-located local earthquakes have been recorded along several profiles through western Nevada, southeast Oregon, and eastern California. Pn delay times from these data and from additional travel times of NTS explosions recorded across the western Nevada seismic network have been interpreted in terms of varying crustal thickness of the western Great Basin. Our interpretation of these data implies that the crust thins from greater than 32 km thick in the vicinity of Mono Lake to 20 to 22 km thick over a broad region in northwest Nevada. The Pn headwave propagates to a distance of about 550 km with a velocity of 7.8 km/sec. At distances greater than 600 km a low velocity (7.44 km/sec), larger amplitude phase is observed. This phase may result from long-period energy diffracting into a shadow zone. A prominent, high-velocity second arrival following Pn by 1 to 2 sec in the distance range 340 to 475 km, is interpreted as a reflection from the base of the lithosphere at 60 to 70-km depth.