ABSTRACT: Arelatively complete section of Eocene (~33.9-55.8Ma) pelagic chalk from offshore northwestern Australia was used to analyze range and abundance data of ~250 Eocene species to test the efficacy of the existingCP(Okada and Bukry 1980) andNP(Martini 1971) biostratigraphic zonation schemes. Changes in nannofossil diversity, abundance, and community structure were monitored through several Eocene paleoenvironmental events, as identified by changes in ä13Candä18Odata, to examine variations in surfacewater conditions.Major changes in nannofossil assemblages, as indicated by dominance crossovers, correspond to paleoenvironmental shifts such as the PETM(Paleocene-Eocene thermal maximum) and the EECO (Early Eocene climatic optimum). This research also provides systematic paleontology and range data for one new genus (Hexadelus) and eight new species or variants (Calcidiscus ellipticus, Cruciplacolithus nebulosus, Cruciplacolithus opacus, Cyclicargolithus parvus, Hexadelus archus, Hayella situliformis var. ovata, Markalius latus, Pedinocyclus annulus) and addresses several taxonomic issues in other Eocene species.