To assess the nature of the heat source associated with the Roosevelt Hot Springs geothermal area, we have investigated the P-wave velocity structure of the crust and uppermost mantle in the vicinity of the Mineral Mountains, southwest Utah, a region of late Cenozoic rhyolitic and basaltic volcanic activity. A roughly square (30X30 km) array of 15 seismographs, centered on the mountains, was operated for a period of 46 days, during which 72 teleseismic events were recorded with sufficient quality for calculation of P-wave traveltime residuals. Relative residuals, using the array average for each event as reference, show a clear pattern of azimuthal variation of up to 0.3 sec. This pattern implies the existence of a localized region of relatively low-velocity material extending up from the upper mantle to depths of about 5 km under the Mineral Mountains. A three-dimensional (3-D) inversion of the data confirms this conclusion and yields a model featuring a region of low velocity (5 to 7 percent less than the surrounding rock) centered under the geothermal area and extending from about 5-km depth down into the uppermost mantle. The near-surface velocities obtained in the inversion clearly reveal the structure of the region, part of the Basin and Range province. An azimuthally changing pattern of wave-form distortion, restricted to the central Mineral Mountains, indicates the presence of a small but intensely anomalous region of low velocity and high attenuation at depths of about 15 km. Although we cannot rule out an explanation for the low velocity purely in terms of compositional changes, in view of the geothermal and volcanic manifestations found in the region we prefer an explanation in terms of abnormally high temperature and a small fraction of heat reservoir than does a model involving only circulation along deep fault zones.