Oxygen isotopic studies both of benthic foraminifera and shallow-marine carbonates have provided a useful monitor of marine palaeotemperatures. The Deep Sea Drilling Project (DSDP) has provided cores from many ocean basins to conduct detailed stable isotopic and palaeoceanographic studies of the Cenozoic and late Mesozoic. DSDP Sites 277 and 292, separated by about 60 degrees latitude in Palaeogene times, each record an (super 18) O enrichment in benthic foraminifera of nearly 1% beginning at the Eocene-Oligocene boundary. Planktonic foraminiferal trends are similar to benthic trends in the high latitude south-west Pacific Ocean, but tropical planktonics show only a minor (-O.3%) increase which may reflect a change in seawater composition. These results suggest a sudden cooling of Pacific deep waters and high latitude surface waters forms a useful stratigraphic marker for the Eocene-Oligocene boundary. This boundary is particularly important because of its association with several worldwide palaeo-oceanographic and biogeographic changes. These include a sudden drop in the calcite compensation depth of 1-2 km; a decrease in planktonic microfossils diversity; a change in planktonic biogeographic patterns; and increased erosion of deep-sea sediments over wide areas.