A comparative study of various sections from western Tethys allows us to define a new refined planktic foraminiferal biozonation valid for low and middle latitudes. The classical zones have been recognized along the Palaeocene-Eocene transition, but high resolution methodology allows us to define new subzones, especially within the Palaeocene-Eocene boundary interval. The planktic foraminiferal faunal turnover across the P/E boundary is gradual with the exception of an acarininid diversification and their excursion toward higher latitudes. Acarinina berggreni is the first acarininid to evolve during the P/E event, whereas the first appearance of other acarininid species (Acarinina sibaiyaensis, Acarinina africana and Acarinina quetra) occurred at or just above a dissolution clay layer present in most of the deep sea sections. The extinction of the bathyal and abyssal small benthic foraminifera occurred just below the dissolution clay interval in the A. berggreni Subzone and constitutes an apparently isochronous datum. The isotopic and planktic foraminiferal excursions began at the base of the Acarinina berggreni Subzone and ended at the top of the Acarinina sibaiyaensis Subzone.