The ECORS Bay of Biscay deep seismic survey was shot in 1984 in co-operation with the ENIEPSA company. It consisted of a 295 km profile running parallel to the Landes coast where vertical and wide angle seismic reflection were recorded together. Six expanding spread profiles providing the seismic velocities and vertical seismic reflection were also recorded. After conventional seismic data processing, the seismic refraction and wide angle data were re-processed with the particular aim of checking the location of the Moho. The geological framework of the Aquitaine region has been compiled. The pre-Triassic basement is characterized by large structural changes from one area to the other. Its development was largely controlled by allochthonous units originated from the north and set on top of the Ebro-Aquitaine micro-plate. The location of the suture bounding this latter unit to the north remains largely debated. Beneath the Parentis basin, the Moho uplift is 15 km and the crust attenuated to a thickness of 5 km. The question of the basin formation is raised and the great crustal thinning, with respect to the extension observed in the basement, leads to suggest non conservative processes.