Teleseismic receiver functions are used to estimate the crustal structure beneath a 36-station, 500-km-long, NW oriented linear array centered on the eastern Snake River Plain and crossing the Yellowstone hotspot swell 250 km SW of Yellowstone. Structure derived previously for this region from wide-angle reflection data is used as an initial model, and this structure explains most features observed in our receiver functions. Based on a combination of forward and inverse modeling, our data require several modifications to the initial structure: (1) Moho depth is ~42 km beneath most of the Snake River Plain, shallows to ~37 km to either side, and thickens abruptly to ~47 km beneath SW Wyoming; (2) a midcrustal layer interpreted previously as a ~9-km-thick gabbroic sill is flat topped across the entire ~90 km width of the Snake River Plain; and (3) a low-velocity layer is found beneath and southeast of the Snake River Plain, which probably is partially molten lower-most crust. Using the seismic structure of the crust to estimate the crustal load upon the mantle, and assuming local isostasy, we calculate that mantle beneath the Yellowstone swell is approximately uniformly as buoyant as 12-million-year-old ocean mantle, and more buoyant than the adjacent Wyoming mantle by an amount equivalent of -1.5 km of elevation. The transition between these regions of greatly different mantle occurs across a major Paleozoic boundary that now separates the Basin and Range from the Rocky Mountains. ¿ 1998 American Geophysical Union