Analysis of 944 single specimens of three species of late Maastrichtian planktonic foraminifera (Racemiguembelina fructicosa, Contusotruncana contusa, and Rugoglobigerina rugosa) from 38 samples spanning the last 3 Myr of the Cretaceous shows consistent isotopic trends through time, consistent isotopic differences among taxa, and high within-sample isotopic variability throughout. Within-sample variability does not change systematically through time for any taxon, but average δ18O values decrease by ~1.5?, and average δ13C values diverge up section. Comparing taxa, average δ18O values are similar within most samples, but average δ13C values generally decrease from R. fructicosa to R. rugosa to C. contusa. In addition, the within-sample variability of individual δ13C measurements is larger for R. fructicosa than for either C. contusa or R. rugosa, an observation which is consistent with a photosymbiotic habitat for R. fructicosa. In terms of Maastrichtian paleoceanography the negative δ18O trend of ~1.5? corresponds to a temperature increase of ~6¿C, and the divergence of δ13C values up section suggests an increasingly stratified water column in the western Atlantic through the late Maastrichtian. We suggest that these trends are best explained by increasing import of South Atlantic waters into the North Atlantic and an intensification of the Northern Hemisphere polar front.