Calcareous nannoplankton assemblages at Ocean Drilling Program (ODP) Site 1259 on Demerara Rise (western equatorial Atlantic) underwent an abrupt and fundamental turnover across the Paleocene/Eocene Thermal Maximum (PETM) approximately 55.5 m.y. ago. The PETM is marked by a dissolution interval barren or nearly-barren of nannofossils due to the rapid acidification of the world oceans. Toweius, Fasciculithus, and Chiasmolithus sharply decrease at the onset, whereas Chiasmolithus, Markalius cf. M. apertus, and Neochiasmolithus thrive immediately after the event, which also signals the successive first appearances of Discoaster araneus, Rhomboaster, and Tribrachiatus. The environmental indications of these changes were further investigated by correspondence analysis on quantitative nannofossil counts. The PETM event has been attributed to CO (sub 2) -forced greenhouse effects. At Site 1259, the elevated pCO (sub 2) and subsequent lowered surface-water pH values at the onset of the PETM caused intensive carbonate dissolution, producing the nannofossil-barren interval. The chemically stressed habitats may well have also induced the evolution of ephemeral nannofossil "excursion taxa", such as Rhomboaster and malformed discoasters (D. araneus and Discoaster anartios). Based on its sudden increase, Markalius cf. M. apertus is considered to have been a local opportunistic species that took advantage of the surface-water changes. At the same time, a presumably higher runoff from continental areas fertilized the western equatorial Atlantic as indicated by an increase in the abundance of r-mode specialists preferring high-nutrient conditions, such as Chiasmolithus, Coccolithus pelagicus, and Hornibrookina arca. Contrasts between the results of this study and previous work at ODP Site 690 in the Southern Ocean, the New Jersey continental margin, and the central paleoequatorial Pacific further demonstrate that the response to the PETM can be influenced by local differences in geologic setting and oceanographic conditions.