The bisexual snail Assiminea grayana Fleming, 1828, is one of the five European representatives of the genus Assiminea Fleming, 1828, and the type species of the genus by monotypy. It lives on the bottom amongst the vegetation in brackish areas, along river estuaries and in similar places near the coasts of the southern North Sea: from the English Channel to the West coast of Jutland and along the Southeast coast of England. The other European species are A. cardonae Paladhile, 1875 (Balearic Islands), A. eliae Paladhile, 1875 (Spain and France), A. ostiorum Bavay, 1920 (Arcachon, France) and A. rufostrigata Hesse, 1916 (Bulgaria and Roumania). In the present note an account of the karyotype morphology and chromosome behaviour during spermatogenesis is given. The study was carried out in the Institute of Genetics, University of Utrecht. Our thanks are due to Prof. Dr. W. J. Boyes (Institute of Genetics, McGill University, Montreal, Canada) for valuable advice on the karyograms; to Dr. M. R. Honer (Department of Zoology, State Agricultural University, Wageningen), for correcting the English text, and to Mr. D. Smit and Miss T. van Wijngaarden for the technical execution of the illustrations in this paper.