Concentrations of Sr, Mg, and Ca were quantified for the shell and septa formed during the final 3 to 4 yr of growth for four species of the extant cephalopod genus Nautilus [N. belauensis Saunders 1981, N. macromphalus Sowerby 1849, N. scrobiculatus (Lightfoot 1786), and N. pompilius Linnaeus 1758]. These data are used to describe several aspects of the physiochemistry of Nautilus in relation to biomineralization of shell and septal aragonite. Species of Nautilus discriminate against the concentrations of Sr and Mg relative to Ca in seawater by 79% and 99.9%, respectively. The power of this discrimination against Sr and Mg varies for shell and septal aragonite within and among species. Although the among-species variation in the Sr and Mg content of shell and septal aragonite is greater than the within-species variation, differences in these concentrations are not unique among all species. However, when the covariance of Sr and Mg is computed as a transformed variable (discriminant function), species can be distinguished with significance (P <0.001) by the combined Sr and Mg chemistry of septal aragonite. Similarly, all species with the exception of N. macromphalus and N. pompilius can be distinguished by the combined Sr and Mg chemistry of shell aragonite.