Tomographic inversions of ~5000 teleseismic P wave travel time residuals image a narrow, deep, low-velocity region centered beneath the eastern Snake River Plain, Idaho. Aligned in the direction of North American plate motion, the eastern Snake River Plain is the locus of time-progressive volcanism leading to the Yellowstone hotspot. The low-velocity anomaly extends to depths of at least 200 km and is flanked by high-velocity mantle to either side. These results are inconsistent with standard mantle plume models which predict a shallow, wide, low-velocity anomaly. By considering the effects of composition, partial melt, and temperature on both P wave velocities and upper mantle densities (assuming local isostasy), we conclude that the low-velocity region is partially molten peridotite and the high-velocity regions on either side are depleted residuum. Swell relief at hotspots has generally been attributed to thermal effects and is commonly used to estimate plume thermal buoyancy fluxes. However, we demonstrate that the compositional effects of mantle melting and depletion can account for at least one fifth and possibly all of the swell relief at Yellowstone. These results suggest that melt generation and segregation are dominant processes within the upper mantle of Yellowstone and allow for the possibility that the fundamental origin of the Yellowstone hotspot is something other than a deep-seated mantle plume.¿ 1997 American Geophysical Union