A detailed study of five representative Maastrichtian sections of deep-water pelagic limestones in the Umbria-Marche Apennines of Italy shows that inoceramids became extinct synchronously throughout this region some 3.5 Ma before the Cretaceous/Tertiary boundary. The decline and disappearance of these bivalves are immediately preceded by two closely spaced peak abundance zones, and are linked with an important change in environmental conditions during the mid-Maastrichtian period, as is indicated by modifications in the delta(13)C composition of sea water and variations in planktonic foraminiferal assemblages. The restoration of stable environmental conditions after this biotic crisis took about half a million years in this region. Excursions of the delta(13)C Signal contemporaneous to the extinction of inoceramids in other Maastrichtian basins throughout the world (i.e. Basque Country and southern oceans) indicate that these bivalves were extremely sensitive to environmental changes. However, accurate magneto-and biostratigraphic correlations between the Italian, Basque, and southern hemisphere sections unambiguously indicate that the extinction of inoceramids was widely diachronous throughout the world of as much as 4 Ma.