The sediments of the late Campanian – early Maestrichtian Bearpaw Formation were deposited during the last movements of the Cretaceous Western Interior seaway in what is now the southern Interior Plains of Canada. Biostratigraphic correlation of the Bearpaw beds in southwestern Saskatchewan is firmly established. The formation carries a rich faunal sequence, including well-preserved ammonites suitable for an oxygen and carbon isotopic study to determine the environment, particularly the temperatures, of the Bearpaw sea. Oxygen and carbon isotopic measurements were made on the aragonitic shells of Baculites rugosus, B. compressus robinsoni, B. cuneatus, B. reesidei, B. baculus, B. grandis, Hoploscaphites nodosus, and H. plenus. The average δ18O values (PDB) from the ammonite zones range from +0.06 to −2.14, with an average of −1.44, which translate into mean temperatures of about 17–27 °C, with an average of 23.2 °C. The δ13C values range from −5.71 to +2.92, with an average of −1.90. Although the δ18O values for the ammonites are consistent with the accepted marine habitat of these molluscs, an apparent correlation between the average δ18O and δ13C values of baculites from each of the zones in the Bearpaw Formation suggests small contributions of fresh water to the Western Interior seaway in southwestern Saskatchewan. Paleogeographical, paleontological, and sedimentological evidence points to such possible contributions as being local and periodic.