Four 300-km-long refraction profiles from western Manitoba and eastern Saskatchewan were shot over the Canadian portion of the Williston basin. On the basis of three distinct seismic velocity intervals, 5.95-6.50, 6.60-6.75, and 7.00-7.61 km/s, the crust in the Nelson Front and the Churchill tectonic province can be vertically separated into three distinct zones. The crust thickens from east to west, reaching a maximum of more than 50 km in the central portion of the basin. The upper mantle velocity increases from 7.84 km/s in the east to 8.47 km/s in central Saskatchewan. Along the eastern flank of the basin, in the transition zone between the Churchill and Superior tectonic provinces, extensive structural variations are interpreted as major faults which appear to distub the entire crust. This thick high-velocity crust beneath the Willison basin indicates that it is not of extensional origin.