The Global boundary Stratotype Section and Point (GSSP) for the Bajocian Stage, formally defined at the base of bed ABII of the Murtinheira section at Cabo Mondego(Portugal), has been ratified by the IUGS. Multidisciplinary biostratigraphical data, mainly based on ammonite and calcareous nannofossil assemblages, assure worldwide correlations; magnetostratigraphic data increase the correlation power. The position of the boundary coincides with the first occurrence of the ammonite assemblage characterized by Hyperlioceras mundum and related species (H. furcatum, Braunsina aspera, B. elegantula). The boundary lies just below the nanno-horizon of the entry of Watznaueria communis and W. fossacincta, and closely corresponds with an inversion to normal polarity correlated with the up-to-date Jurassic magnetic polarity time scale. An Auxiliary Stratotype Point (ASP) is also selected at the base of bed U10 of the Bearreraig Bay section on the Isle of Skye, Scotland, as the complementary reference for the palaeobiological key of the Bajocian lower boundary, i.e. the evolutionary transition Graphoceras-Hyperlioceras.