Based on evaluation of geochronological data of Cainozoic magmatic manifestations in the Tyrrhenian basin and the surrounding lands, the episodic nature of the volcanism is recognized and an hypothesis on the existence of two phases of formation of the oceanic crust in the bathyal plain is discussed. The phases would have formed, respectively, a central bathyal plain and a south-eastern bathyal plain during distinct processes of asthenosphere updoming. The early process would have taken place in the Lower Pleistocene time, about 7-8 million years ago, and the late one, presumably, during the Upper Pleistocene between 1.7 and 1.2 Ma. The two phases of oceanic accretion seem to have occurred during times of quiescence of volcanic activity in the bordering land areas. The hypothesis is in agreement with the existence of an important south-west-northeast structural discontinuity through the bathyal plain indicated by geophysical investigations of various type. According to this analysis of time-space evolution, the inferred sequence of volcanic events in the Tyrrhenian Sea, considered a marginal basin that formed behind the Apenninic chain and foredeep, shows analogies to that recognized in other marginal basins; for example, in the interarc and backarc basins of the Philippine Sea. Owing to the insufficient knowledge of the Tyrrhenian volcanism, these considerations are to be regarded as working hypothesis.