Deep crustal features of the transition between the North Iberian mainland and the Bay of Biscay are constrained from new wide-angle seismic and gravity data. Velocity-depth models are derived from far-offset recordings onshore of two N-S marine profiles, complemented with new refraction data inland and tested against models that fit the gravity anomalies. Important lateral variations are inferred along the North Iberian margin. In the western transect, a Variscan crust is documented from the mainland across the continental shelf, and a rapid crustal thinning beneath the slope marks the transition to the oceanic domain. The eastern transect shows an outstanding crustal thickening beneath the coastline (Moho depths around 50 km) and a gradual thinning across the shelf. Differences are related to the V-shape of the Bay of Biscay that causes a W-E transition along the margin, from a short-lived oceanic subduction to continental collision during the Tertiary oblique convergence between Europe and Iberia. ¿ 1998 American Geophysical Union