The main aim of this study was to reveal reproduction barriers and potentially cryptic radiation within the very common and morphologically variable leech species - Erpobdella octoculata (L., 1758). The differences in reproductive success of the morphological forms were compared experimentally. The data based on the results of field sampling and analysis of ITS and COI sequences is also presented. The results of the analysis of DNA sequences clearly show lack of reproduction barriers between the analysed morphological forms. Subtle differences in DNA sequences between individuals, found with the use of the arbitrary primers method, seem to be related mainly to geographical distance between sub-populations. On the basis of these results, E. octoculata should be treated as one, valid species, in spite of its great morphological variability. However, experimentally confirmed differences in reproductive success of individuals belonging to morphologically divergent forms may be explained by the presence of reproductive barriers between them. Visible incongruity between the results of DNA sequences and the experimental results is not easy to explain - this question definitely needs further studies. Different percentages of morphological forms noted in field samples, suggests an adaptive value for divergent colour patterns in different habitats. Such differences may be a result of diverse mortality of individuals differing in colour dwelling on different bottom substrate, caused by predation by benthivorous fish.