The specificity of the association between the freshwater leech Placobdelloides jaegerskioeldi and the hippopotamus was confirmed both by collections from the Kruger National Park and by laboratory experiments. Placobdelloides jaegerskioeldi is established as the only leech showing species specificity for a mammal. Forty sexually mature leeches were collected free-living and 2260 mature and immature leeches were collected from 40 of 53 culled hippopotami. In the field P. jaegerskioeldi was never found on any of the alternative hosts, and none fed in the laboratory when offered a range of potential hosts. A combination of unique attributes, including mode of locomotion, anatomical features, and feeding behaviour, makes it possible for this leech to utilize the hippopotamus as a host. Evidence suggests that mating in P. jaegerskioeldi is restricted to the rectum of the hippopotamus.