The mean annual range of temperature is, from a climatological point of view, at least as important as the mean annual temperature. In order to examine intra-annual temperature variations in the warm early Cenozoic, we have analyzed stable oxygen and carbon isotopes along the transect of growth on excellently preserved specimens of the gastropod Turritella imbricataria and the bivalve Venericardia imbricata from sedimentary rocks of early middle Eocene age (ca. 46 Ma) in the Paris basin. Turritella shell growth is fast throughout the year, and the isotopic records give the first detailed information about early middle Eocene seasonal variations. The oxygen isotope profiles indicate winter temperatures of about 14 °C and summer temperatures of up to 28 °C, i.e., a seasonality indistinguishable from the present situation but an annual temperature some 10 °C higher. Our results contradict the opinion that the climate during this period was more “equable” than that of today.