Fundamental mode Rayleigh wave (4- to 120-s period) and Love wave (4- to 40-s period) phase velocities have been measured for several paths wholly within the Great Basin of Nevada and western Utah. An average structural model for this area was derived from the dispersion data and existing refraction data. This model consists of a three-layer, 35-km-thick crust and a 29-km-thick, higher-velocity (Vs=4.5 km/s) cap at the top of the mantle, below which lies a broad (180-km-thick) low-velocity zone for shear waves (Vs=4.05-4.12 km/s). Intermediate-period (15- to 32-s) Rayleigh wave phase velocities measured over shorter paths indicate the existence of significant local variations in the shallow (20- to 30- km-deep) structure which may result from varying crustal thickness or varying depths to a zone of partial melting. Comparison of our Great Basin mantle structures of ther areas (oceanic, shield, alpine, and rise) shows greatest similarity with the East African Rift and the East Pacific Rise. Like these areas the Great Basin may be a tensional region where deep mantle material has intruded near the surface and created a zone of partial melting.