In a paper on the validity of Vallonia excentrica Sterki, HUBENDICK (1952) proceeding from MAYR’s definition of a species concluded that the species concept is biological and not morphological. Two species should, therefore, be separated biologically, in other words by breeding experiments. This is, however, very difficult particularly in hermaphrodites where self-fertilization cannot be excluded. PARAENSE (1956) proved in a fine study, working with albino markers, that this method is very certainly feasible. In many other cases involving experimental crossings of critical species it must be kept in mind that a fertile offspring does not permit one to lump the two parental nominal species since the critical question still remains: when two “species” are able to produce fertile offspring in the laboratory, do they freely interbreed in their natural biotope? We cannot, in any case, do without morphological methods and we must consider them in relation to all other data available. We now return to HUBENDICK’s advice: “If therefore a morphological method must be applied, the most certain way and undoubtedly the most correct way is to do chromosome studies”. BURCH (1964) concluded from a study on chromosomes in Oncomelania that three genera, 19 species and 2 subspecies are in reality only one species with several geographical races. It has always been very difficult to discriminate between Hydrobia ulvae (Pennant, 1777) and Hydrobia stagnorum (Gmelin, 1790) the latter being sometimes understood as an ecotype of the former species (SPAINK, 1961). In this connection the validity of the Danish Hydrobia neglecta Muus, 1963, was doubted. From this point our interest in molluscan chromosomes was awakened and we started our research by collecting data from the literature on the cytology of the family Hydrobiidae. The literature dealing with European species is reviewed in the next pages. We thought it useful to mention data on the period of egg production and copulation as there is a better chance of finding good cell divisions in animals collected prior to that period.