Specimens of Nautilus species caught in the wild show a marked increase in oxygen isotopic composition between embryonic and postembryonic septa. The significance of this increase in terms of the early life history of Nautilus has been unclear. To help explain this pattern, we analyzed the isotopic composition of the septa of three specimens of Nautilus belauensis raised in aquariums under controlled temperature conditions. Our results indicate that both embryonic and postembryonic septa are secreted with the same temperature-dependent fractionation of aragonite relative to water as that of other aragonite-secreting molluscs (Grossman and Ku 1986). The δ18O values of the septa thus provide a reliable means of determining the water temperature in which the septa form. Calculated temperatures based on oxygen isotopic data from specimens caught in the wild reveal that embryonic development occurs at 22°-24° corresponding to a depth of 100-200 m depending on the location. The increase in δ18O in postembryonic septa reflects a migration into colder, deeper water after hatching. In Cretaceous nautilids, a systematic shift in δ18O is not present, indicating that these animals probably did not change their habitat after hatching. This is consistent with the likelihood that they lived in shallower environments than that of modern Nautilus.