The 1978 Yellowstone-Snake River Plain seismic experiment provided detailed refraction data that were recorded across a two-dimensional array of seismographs in Yellowstone National Park. A delay time analysis was applied to 173 crystalline basement Pg arrivals from these data to determine the three-dimensional distribution of velocities and the layer configuration of the upper crust beneath the Yellowstone caldera. The P wave velocity structure of the caldera is characterized by a surface layer of combined sediments and rhyolite flows, averaging 2.8 km/s, that range in thickness from 1.5 to 2.0 km. Adjacent to the caldera, the crystalline upper crustal layer has a velocity of 6.05¿0.01 km/s, but this layer decreases by 6% to 5.70 km/s beneath the caldera and extends northeast 15 km beyond the caldera. Smaller zones of very low P velocity, 4.0 km/s, a 30% velocity reduction compared to the 6.05 km/s layer, occur in the upper crust beneath the northeastern caldera rim and beneath the southwest caldera in the vicinity of the Upper and Midway Geyser basins. A three-dimensional gravity interpretation based upon densities derived from the seismic model suggests that the regional gravity low of -60 mGal over the caldera correlates directly with (1) the surface layer of combined sediments and rhyolite flows, (2) the low-velocity, 5.7-km/s, upper crustal layer, and (3) the 4.0-km/s low-velocity zone beneath the northeastern caldera rim. An interpretation of the seismic velocities and densities, based on experimental data and theoretical models, suggests that the 6.05-km/s (&rgr; = 2.70 g/cm3) regional Pg velocity is characteristic of crystalline basement rocks such as the Precambrian granitic gneisses that are exposed north and south of the Yellowstone caldera. The 5.7-km/s velocity layer (&rgr; = 2.65 g/cm3) is interpreted to be a 10-km-thick hot body of granitic composition. The 4.0-km/s low-velocity body (&rgr; = 2.40 g/cm3) beneath the northeastern caldera rim is associated with a local -20 mGal gravity anomaly with a range of interpretations from a large steam-dominated system to a felsic body of 10 to 50% partial melt. The postulated 4.0-km/s low-velocity layer beneath the southwestern caldera does not have a corresponding gravity signature and may reflect a zone of high seismic attenuation attributed to a saturated fracture zone. This seismic velocity model of the Yellowstone caldera suggests that the source of the Quaternary silicic volcanic rocks corresponds to the 5.7-km/s layer in the upper crust that appears to contribute the principal source of heat to the Yellowstone hydrothermal systems. The anomalous low-velocity body in the northeastern caldera rim area may be a cupola of partial melt with an overlying vapor-dominated carapace and may represent the waning stages of the third cycle of quaternary volcanism or the beginning of a new cycle of volcanics that will continue the northeastward progression of the silicic volcanism of the Yellowstone-Snake River Plain propagating system.