We have measured fundamental mode Rayleigh wave phase velocity dispersion from seismograms of five earthquakes recorded at stations on the islands of Hawaii and Oahu and inverted these data for upper mantle velocity structure. The seismic lithosphere of the velocity model is 88¿7 km thick, which is similar to that of 80--90 Myr oceanic lithosphere, indicating that no significant lithospheric thinning takes place above the mantle plume. Below the lithosphere the shear wave velocity decreases to ~4.0 km s-1. The seismic model is consistent with the structure of the Hawaiian plume model of Watson and McKenzie [1991> which has a 72-km thick mechanical boundary layer with the shallowest melting occurring at 82 km depth. The difference between the dispersion measured on the Hawaii-Oahu path and that observed by Woods and Okal [1996> on the Oahu-Midway path can be explained by the cooling of the injected hot plume material. ¿ 1999 American Geophysical Union