A large, interesting otolith fauna from the Middle Oligocene was found in the clay pit 'De Vlijt' near Winterswijk by sifting large quantities of clay and by collecting large loose specimens in the clay pit. Thirty-five species and one subspecies were indentified; nine species are new, viz. Clupeiformorum planus, Congridarum trapezioides, Gadichthys altus, Palaeogadus compactus, Brotulidarum phaseoloides, Paralabrax splendens, Dentex kokeni, Carangidarum robustus, and Acropomidarum martini. The study of part of Koken's type material was very helpful for the identification of many species. Photographs have been included of important specimens in Koken's collection. The fauna, in which Argentinidae, Gadidae, Brotulidae, and Ophidiidae dominate, is indicative of a moderately warm to temperate sea with a probable depth of 100-200 m. The Middle Oligocene North Sea surely must have been in open communication with the Tethys Ocean.