Although the European medicinal leech (Hirudo medicinalis L. 1758) is one of the best-known members of the Hirudinea due to its use in phlebotomy, this species has been confused with the Mediterranean taxon H. verbana Carena 1820. Here we describe the morphology of adult and juvenile H. medicinalis and document its genetic distance to H. verbana, using newly acquired mitochondrial DNA-sequence (cytochrome c oxidase subunit I, CO-I)-data from specimens collected in Germany. Our CO-I analysis shows that H. medicinalis and H. verbana differ by 9.4 %. Hence, the original Hirudo-population diverged ca. 10 million years ago so that today two geographically separated biospecies exist that co-occur in only a few natural habitats. We analyzed the behaviour of adult H. medicinalis, but could not find differences with respect to its sister taxon H. verbana. Finally, we summarize the occurrence of H. medicinalis in Central Europe and conclude that this once widely distributed freshwater species largely disappeared in many countries. We suggest that the loss of natural freshwater ecosystems, with flat, warm banks, and amphibians (frogs, newts and toads) as preferred host organisms for the juveniles, are largely responsible for the decline of H. medicinalis in Northern Europe.