Quantitative analysis of organic-walled dinoflagellate cysts (dinocysts), foraminifera and calcareous nannofossils from boreholes in Belgium and Germany enables the recognition of biotic events and reconstruction of environmental change in the late Rupelian and early Chattian in their type region the southern North Sea Basin. Compared to the early Rupelian depositional conditions (neritic) do not seem to have substantially changed during the late Rupelian. The microfossil groups died indicate relatively cold-water conditions in an outer-shelf environment. The boundary between the (lower) Rupelian stratotype section and the Upper Rupelian subsurface succession can be recognized by the first occurrence (FO) of Saturnodinium pansum and the last occurrence (LO) of Enneadocysta pectiniformis (dinocysts). The traditional Rupelian-Chattian (R-C) boundary definition, at the base of the benthic foraminiferal Asterigerinoides guerichi acme known as the Asterigerina Horizon, is upheld here. In terms of dinocyst biostratigraphy, it coincides with the FO of Artemisiocysta cladodichotoma and the recurrence of Pentadinium imaginatum, falling within the middle of the NP24* nannofossil zone (* points to the substitute zonation for the North Sea Basin). The traditional R-C boundary coincides with an abrupt return of shallow warm-water conditions and represents a third-order sequence boundary. The R-C boundary is overlain by transgressive Chattian deposits, which correspond to a distinct warming event. This pulse may correlate with the globally detected Late Oligocene Warming Event, which has an approximate age of 26 Ma. A mid-Chattian hiatus coincides with the NP24*/NP25* boundary and with the simultaneous disappearance of several dinocyst taxa. A detailed literature study proved that the LO of the planktonic foraminiferal genus Chiloguembelina is globally diachronous. Hence, the validity of this Chiloguembelina extinction as a global R-C boundary criterion can be questioned. This study offers new perspectives for the search for a R-C boundary Global Stratotype Section and Point (GSSP). (C) 2004 Elsevier B.V. All rights reserved.