Two-station measurements of Rayleigh wave group velocity between Midway Atoll and O'Ahu provide a useful constraint on lithospheric thickness along the Hawi'ian Swell. Comparison of the observed dispersion curve with age-dependent, regionalized dispersion curves suggests that the swell acts as 50--110 m.y. old lithosphere, and not as 20--50 m.y. old lithosphere as is predicted by the currently favored hypothesis for the swell's formation. A preliminary isotropic shear velocity model for the swell suggests that the lithosphere may be as thick as 100 km. This value is difficult to reconcile with the lithosphere being substantially thinned during its passage over the Hawai'ian hotspot. ¿American Geophysical Union 1991