The oxygen (delta 18 O) and carbon (delta 13 C) isotope values from marine foraminifera indicate a global climatic cooling and significant changes in oceanic circulation from the late Eocene to the early Oligocene. The carbon and oxygen isotope composition of ungulate tooth dentine and enamel carbonate hydroxylapatite from French Eocene and Oligocene localities was measured in order to use the isotopic analyses to better understand the palaeoenvironmental changes at the Eocene/Oligocene boundary and to tentatively establish a correlation with curves established from marine delta 13 C and delta 18 O values. The variations of delta 13 C and delta 18 O values are important between the different species, as among modern mammalian species, but a 1 to 2 per mil difference exists between fossil and modern forms. The tooth enamel carbonate hydroxylapatite from Palaeogene ungulates seems to have suffered some degree of diagenesis. Part of the biological isotopic signal may be diagenetically altered. The degree of resolution in the studied Palaeogene mammals is not sufficient to interpret palaeodiets in C 3 -plant palaeoenvironments or to establish correlations with marine carbon and oxygen isotopic curves. However, carbon and oxygen isotopic fluctuations from ungulate tooth enamel seem to follow the same direction as marine curves.