Crustal thickness determinations were made from seismic refraction measurements along two profiles extending north–northwest along the Inside Passage of Southeast Alaska and western British Columbia. The source for the seismic waves was a series of 10 explosions detonated by personnel of the Earth Physics Branch, Department of Energy, Mines and Resources of Canada. Arrivals recorded in Southeast Alaska from explosions in Bird Lake on Graham Island indicate a 26 km thick crust over a mantle with a Pn velocity of 7.9 km/s. Measurements of Pn wave attenuation suggest that the uppermost mantle has a Q value of 650 ± 120. Arrivals recorded along the mainland coast of British Columbia from explosions in Ripley Bay indicate a 30 km thick crust over a mantle with a Pn velocity of 8.1 km/s. On the basis of compiled gravity and seismic refraction data, the crust of the Inside Passage region from Queen Charlotte Sound to Cross Sound is believed to be anomalously thin. Crustal thickening from ocean to continent appears to occur in a stepwise, rather than continuous, fashion in the vicinity of the Inside Passage.