Group velocities of Rayleigh waves recorded at a long-period seismograph station on Amchitka Island were obtained for mixed tectonic paths across the Pacific and the Aleutian Island Arc Ridge. The mixed-path group velocities for periods between 20 and 60 sec were then separated into pure-oceanic and pureridge path group velocities. The group velocities for the pure paths along the Aleutian ridge are on the average 0.36 km/sec lower than those for the purely oceanic paths across the Pacific. Inversion of the pure-ridge group velocities yields almost continental shear velocities in the crust, a very gradual crust-mantle transition at depths between 20 and 40 km, a thin lithospheric lid of uppermost mantle material between 30 and 70 km with relatively low maximum shear velocities approaching 4.4 km/sec, and a very pronounced low-velocity zone at depths below 70 km with an average shear velocity of 4.1 km/sec. The computed shear velocity structure beneath the Aleutian Ridge is compared to models for other tectonically active and stable regions.