A dominant feature of present-day thermohaline circulation in oceans is the production of cold Antarctic Bottom Water (AABW) in the Weddell and Ross Seas and along the continental margin off Antarctica. The cold, dense AABW formed at or near the surface sinks to abyssal depths and travels throughout the ocean basins. AABW circulation was suggested to have begun near the Eocene/Oligocene boundary with the development of the psychrosphere (lower cold layer of a two-layer ocean with temperatures <10°C) inferred from ostracode faunal data1 and oxygen isotopic evidence from closely-spaced samples in DSDP site 277 (ref. 2). Previous studies suggested that the temperature change associated with the development of the psychrosphere caused a crisis for deep-sea ostrocodes1 and deep-sea benthonic Foraminifera3. I consider here the response of benthonic Foraminifera to the development of the psychrosphere. © 1979 Nature Publishing Group.