The variation with time in the oxygen isotope ratio of the oceans is of prime interest in a variety of research fields. An excellent correlation between oxygen isotope ratios of cetacean (whales, dolphins, and porpoises) bone phosphate and their environmental water is found in this study. Bone phosphate samples of dolphins living in fresh waters are more depleted in oxygen 18 than those of cetaceans living in the oceans, reflecting the clear difference in the isotope composition of water. Cetaceans distributed in the higher latitudes in the oceans are more depleted in oxygen 18 than those distributed in the lower latitudes where seawater is slightly enriched in oxygen 18 relative to that in the higher latitudes. The present results provide a promising tool for estimating the oxygen isotope ratio of the oceanic water of the past without assuming water temperature.