Oxygen and carbon isotope ratios are reported from exceptionally well preserved, early Late Cretaceous (Cenomanian) aragonitic mollusks (ammonites, bivalves, gastropods) from the Moonkinu Formation exposed on the southern coast of Bathurst Island, northern Australia. Samples were examined by means of X-ray diffraction and electron microscopy to screen for diagenetic alteration. The oxygen isotope data derived from planispiral ammonites (largely Euomphaloceras and Acanthoceras), interpreted in terms of temperature, provide the warmest temperatures, ranging up to 34 °C, in accord with the subtropical paleolatitude and with other temperature estimates associated with the middle Cretaceous thermal maximum. Oxygen isotope data from benthic mollusks suggest shelf bottom-water temperatures of ∼21 °C. The oxygen isotope data derived from the straight-shelled baculitid heteromorphic ammonite Sciponoceras are consistent with these organisms having a nektobenthic mode of life, differing from previous views of Baculites which suggest habitation of the mid to upper parts of the water column. Isotopic analyses of shells of the inoceramid bivalve (Actinoceramus) in general show relatively positive carbon values and the most negative oxygen isotope values. Such values are more characteristic of surface-dwelling organisms, and approach those we have measured for planispiral ammonites. Isotopic data from other benthic organisms including the gastropod Latiala and other bivalves provide δ¹⁸O data that are consistent with a normally stratified water column. We view the isotopic values for Actinoceramus as anomalous relative to expected values for the watermass in which the animals lived, and ascribe them to taxon-specific disequilibrium or a vital effect as the cause of depleted oxygen isotopic values that are characteristic of inoceramids in general.