Until now Schistosoma mansoni has been regarded as an essentially human parasite. In 1859, Cobbold recorded from Cercocebus fuliginosus a species of Schistosome which he called Bilharzia magna. The only specimen which has been preserved was found to be a fragment of a male by Leiper (1915), who states, “I have been quite unable to identify it with either of the species now recognised in man.” Various authorities have, however, referred this species to S. hœmatobium. S. mansoni has been grown experimentally by Leiper in rats, mice, guinea pigs and African and Indian monkeys, and by Lutz in rabbits. It has never been found naturally in any animal other than man.