Eleven in-line refraction profiles, recorded to distances of 300 km, and one azimuthal fan plot were constructed from data recorded with a 150-station array in the Yellowstone National Park area during the 1978 Yellowstone-Snake River Plain seismic experiment. Interpretations of the data suggest that the crustal P wave velocity model for the Yellowstone region is characterized by (1) an averaged 10-km-thick upper crustal layer, Vp = 6.0 km/s, (2) an average crustal velocity of 6.3 km/s, and (3) a total crustal thickness of 44 km. Velocity models are presented for profiles that emphasize the upper crust and show (1) a decrease in the depth to the top of the upper crustal crystalline basement from 5 km in southwestern Yellowstone near Island Park to 1 km at the northeast side of the Yellowstone Plateau that is interpreted as a progressive thinning of the silicic surface volcanic layer to the northeast and (2) evidence for a large lateral inhomogeneity interpreted to be a low-velocity body, with a decrease of at least 10% in P wave velocity, located beneath the northeast corner of the Yellowstone Plateau. The low-velocity zone coincides with a local -30-mgal residual gravity anomaly and is located beneath part of the Sour Creek resurgent dome and part of the Hot Springs Basin, the largest hydrothermal system in Yellowstone. The low-velocity body has a maximum depth to the top of 3 km and a minimum depth to the bottom of 9 km and may represent a zone of partial melt. In comparison to the thermally undisturbed upper crust of the surrounding Rocky Mountains the upper crust of the northeastern Yellowstone plateau appears laterally inhomogeneous in velocity and layer thickness, suggesting effects of thermal and magma intrusion, whereas the lower crust appears relatively homogeneous.