Six OBS deployed on a 550 km line in the Tyrrhenian sea furnish reversed and overlapping profiles from 43 shots at 12 km intervals. Head waves recorded from a 8 km/s refractor, the Moho, permitted the computation of the crustal thickness, which shows this basin to be underlain by an oceanic type of crust, at least in two 100 km wide regions. Variations of Pn arrival times mirror strong topographical changes of the Moho. Differences in dips at the extremities towards Corsica and Calabria are documented.