New seismic refraction measurements made with radio-sonobuoys, ocean bottom seismographs, and ship-deployed hydrophones are used to improve the knowledge of structure of the Colorado Basin and the nature of the boundary between continental crust and oceanic crust. Included are the tabulated results of 120 airgun-sonobuoy stations on the continental shelf, slope, and rise off Bahia Blanca and a schematic (seismic) structure section of these features. The seismic data and information from boreholes on the shelf are interpreted to show the presence of an outer shelf-slope basement ridge, perhaps capped by a volcanic or reefal complex, that may have barred marine deposition from the Colorado Basin until the close of the Cretaceous. The ridge of continental basement rock is lo ated beneath sediments of the upper continental slope, about 80 km beyond the edge of the shelf, at the boundary between continental and oceanic crust. Closely spaced refraction profiles made parallel to isobaths in the vicinity of the continent-oceanic crust boundary suggest stretching and thinning of lower crustal rock, perhaps accompanied by subsidence of continental basement rock to form the continental slope. Alternatively, as is suggested by others to explain a coincident belt of magnetic and gravity anomalies in the vicinity of the outer shelf-slope, the basement of the continental slope is elevated material of composition similar to oceanic crust.