The monogenetic trematodes have long been known as parasites of the lower aquatic vertebrates and many forms have been described from fishes, amphibians, and reptiles. They are largely ectoparasitic, although a few members of the group infest the oronasal cavities and urinary bladder of frogs and turtles. Up to the present there appears to be no record of their occurrence on either birds or mammals and consequently the unusual case here reported seems worthy of note. The material consists of five polystomes which bore the label, “from the eye of hippopotamus.” Presumably they were collected by the late Professor A. Looss from a Nile hippopotamus in the Giza Zoological Gardens of Cairo, Egypt, but unfortunately no particulars are available. The specimens were given to me for identification by Dr Edward Hindle, recently Professor of Biology, School of Medicine, Cairo, to whom I wish here to express my thanks. Grateful acknowledgments are due also to Professor George H. F. Nuttall, Director of the Molteno Institute for Research in Parasitology, where the study was made.