Thirty-three seismic refraction profiles have been obtained in Lake Superior using a single ship sonobuoy method. The seismic data have led to the following conclusions concerning the structure and stratigraphy of the late Precambrian Keweenawan basin that underlies the lake:(1) Faults with their north side downthrown at least 1–2 km, bound the north shores of Isle Royale and Michipicoten Island.(2) The northern limb of the Keweenawan basin just southeast of Isle Royale appears to have undergone a deformation that may be partly related to movement along the Isle Royale fault.(3) For those seismic profiles that have a certain degree of geological control, refraction velocities generally agree well with those estimated from sample measurements in the laboratory.(4) Late Keweenawan or early Cambrian Bayfield-Jacobsville sandstones appear to underlie most of Lake Superior. Their distribution and relative susceptibility to erosion compared with older rocks are the principal factors that have determined the shape of the lake depression.