Two-station methods are employed to determine Rayleigh wave phase velocities along three paths crossing the Afro-Arabian dome. Phase velocities are modeled using a grid-search technique, and the range of successful models indicate that the uppermost mantle beneath Afar and western Saudi Arabia is characterized by pronounced negative S wave velocity gradients and S wave velocities that are 0.2--0.8 km/s lower than PREM at 100 km depth. These results are consistent with a thermal plume model for Afar, provided that there is a few percent of partial melt present within the mantle. The low velocity anomaly beneath western Saudi Arabia could be caused by migrating plume material from Afar, upwelling mantle material associated with Red Sea rifting, or isolated small-scale convection. ¿ 1998 American Geophysical Union