Temporal changes in the oxygen isotopic composition of specimens of fossils (mainly the bivalves Macoma and Pecten) from four sections of Neogene rocks exposed in the Eel River Basin of northern California resulted from changes in temperature and global glacier volume. The temperature was higher (probably 10–15 °C) during the early history of the basin than later. The temperature was probably 5–10 °C during much of the Plio-Pleistocene and showed no well-defined trends, even though the depth of deposition decreased considerably during that time interval. This uniformity was due to strong upwelling of deep water that kept the shallow-water temperatures low. Samples from shallow-water and relatively inland settings at Holmes and Garberville have lower 18O/16O ratios than do samples from the other two sections. This is because of either higher temperature of these shallow waters or dilution of the sea water by isotopically light fresh water. The difference in fractionation of oxygen isotopes between calcite (Pecten) and aragonite (Macoma) fossils potentially provides a means of determining oxygen isotope–based paleotemperatures that is independent of the isotopic composition of the water in which the animals lived. The carbon isotopic composition shows a vital effect and is more variable in the nearshore samples.