Comparison of oxygen isotope data for exceptionally well preserved cooccurring plankton and benthos from the Campanian of Hokkaido, Japan, with nine species of ammonoids clearly indicates the demersal (nektobenthic) habitat of ammonoids; unlike Nautilus, the ammonoids studied did not engage in significant short-term vertical migrations in the water column. The new foraminiferal isotopic data suggest that sea-surface and sea-bottom temperatures were ∼26 and 18 °C, respectively, at 40°N in the Campanian northwestern Pacific. The temperatures were significantly warmer than those in the modern northwest Pacific. This finding provides the first reliable evidence for the warm Late Cretaceous mid-latitude North Pacific. Isotopic analyses of ammonoids show that the average calcification temperature of all ammonoid shells analyzed was ∼19 °C, comparable to those of cooccurring benthos. None of these ammonoids display calcification temperatures equivalent to those of planktonic foraminifers.