Chromosomal data are given for five European species of Anacaena. A. bipustulata, A. limbata and A. lutescens have the diploid number 16 + XY (male), 16 + XX (female). A. globulus has the number 14 + XY/XX, while in A. rufipes the number is 10 + neo-XY (male). Chromosomal data from various Hydrobiinae suggest that a diploid number of 16 + XY or XX is normal in this subfamily, and that the karyotypes of A. globulus and A. rufipes result from reductions in number. In A. globulus the sizes and shapes of the chromosomes suggest that there may have been loss of the centromere from one of the smaller chromosome pairs, with the two arms fusing with two different autosomes. In A. rufipes the process appears to have gone further, with the original sex chromosomes fusing with a pair of autosomes to give a neo-XY. The karyotypes of all five species are distinctive, confirming the recognition of A. lutescens as a distinct species. In populations of A. lutescens comprising only females, all specimens were heterozygous for a deletion polymorphism in one pair of autosomes. This deletion was not found in bisexual populations. Two of the females-only populations included a proportion of triploid individuals, and this, in conjunction with the deletion polymorphism, is taken as evidence that these populations are parthenogenetic. Present evidence is not considered sufficient to show whether the parthenogenesis has a single origin in A. lutescens or whether it is polyphyletic, though the discovery of consistent small differences in the relative length of two autosomes, between parthenogenetic and bisexual populations, suggests a single origin. The triploid karyotypes show small differences between the two populations in which they are known, and this suggests that triploidy has arisen after the establishment of parthenogenesis, and is of multiple origins. In most cases the only tissue available for chromosome preparations was mid-gut of adult beetles, and the methods used are described.