Extensive paleomagnetic sampling of 38 late Miocene and Pliocene sites has been carried out in the western Tuscan basins in Italy. These basins are located along the Tyrrhenian margin, and the studied formations were deposited at the same time as the opening of the Tyrrhenian Sea. The laboratory analyses show that, in most of the sites, pseudo-single-domain magnetites are the main magnetic carrier. The paleomagnetic data obtained from 27 suitable sites indicate that the Tuscan Tyrrhenian margin has not been affected by significant regional rotation since at least the late Messinian-early Pliocene (about 5.5 m.y.). These data demonstrate that the rotations previously measured in the Meso-Cenozoic units of the Apennines and of the Apulia foreland cannot be directly related to the late Messinian-Pliocene phases of rifting in the Tyrrhenian Sea. Moreover, the data presented here conflict with published results from coeval formations in the Apennines and Sicily, in areas subjected to strong thrusting and shortening. In these areas the rotations control the orientation of the main compressional structures and give rise to the typical arcuate shape of the Apenninic chain. We suggest that the tectonic evolution of the northern Tyrrhenian Sea-Apenninic chain system involved thrusting and shortening of the external (eastern) domains, with rotations of the allochthonous units, followed by gravitational collapse of the western sector. This second phase involved no tectonic rotation of the newly formed extensional basins. ¿ American Geophysical Union 1996