The Bathonian is an interval from which salinity-controlled benthic faunas are particularly well known. This study examines Late Bathonian faunas from Dorset, UK, from the Wattonensis Beds (Fuller's Earth), of known marine origin, the overlying Elongata Beds, and the Boueti Bed marking the base of the Forest Marble. δ18O values of belemnites (Belemnopsis) from the Wattonensis Beds range from −1.4 to −0.7‰ and carbon isotope values range from −2.3 to −0.5‰ VPDB. Isotope values of well-preserved Rhynchonella from the Wattonensis Beds range from −2.4 to −1.3‰ for oxygen and −1.1 to 0.6‰ VPDB for carbon. The δ18O data suggest high palaeotemperatures (possibly up to 28 °C). A similar spread of data is derived from brachiopods (Goniorhynchia boueti) from the Boueti Bed. Given the marine origin of the Wattonensis Beds and overlap of data, the reduced faunal diversity of the Boueti Bed is considered unlikely to be the result of reduced salinity. Brachiopods therefore should not be regarded more sceptically as evidence for fully marine conditions, as has been suggested. The oysters (Praeexogyra hebridica elongata) and brachiopods (Rhynchonella) from the Elongata Beds have isotope values that also imply fully marine conditions and are interpreted as being indicative of slightly cooler conditions.