The results of a precise and homogeneous aeromagnetic survey covering the Bay of Biscay, France, and the North Western Mediterranean are presented. The results suggest that the Bay of Biscay and the Mediterranean basin were created since the Triassic by drift of the adjacent continents. Indications are given about the times and geometries of opening of these basins. The implications of these results on the Mesozoic and Cenozoic tectonic history of South-Western Europe are important. [NOT CONTROLLED OCR]