Bithynia troschelii (Paasch, 1842) and B. leachii (Sheppard, 1823) from 3 and 4 German localities, respectively, were studied by means of cellulose acetate gel allozyme electrophoresis, to compare 23 enzyme systems represented by 34 loci. In none of the loci a fixed difference was found between the two taxa. B. troschelii is much bigger than B. leachii; it also differs from the latter in the flagellum/penis length ratio. B. troschelii is rarer and its range narrower than that of B. leachii, in which it is contained. The results suggest a very close relationship between the taxa, but due to the striking size difference, representatives of one taxon could hardly mate with those of the other, so their reproductive isolation should be efficient. Thus B. troschelii is most probably a distinct species separated from B. leachii very recently, probably by saltational speciation.