Seismic refraction measurements show that in the western part of the Bering Sea (the Kamchatka or Komandorski basin) the sediment is thinner than in the eastern basins of the Bering Sea but much thicker than the Pacific basin average. The basement velocity (V=5.5 km/s) is within the normal range for oceanic stations, and the basement is thicker than normal (3.1 km). The principal crustal layer (V=6.8 km/s) is thin (2.6 km), and the mantle velocity (V=.0 km/s) is low. A small anisotropy of mantle velocity is observed when the light-velocity direction is nearly east-west, perpendicular to the trend of the magnetic anomalies. East-west spreading is indicated by both the anisotropy measurements, which measure lineations in the uppermost mantle, and the magnetic measurements, which measure lineations in the basement. If there has been extension in this marginal basin, the extension has been parallel to the bordering trench rather than perpendicular as in other marginal basins.