Summary A review is presented of the species of the Heterophyidae known to infest man. They have been placed in the four genera, Heterophyes, Metagonimus, Stamnosoma, and Monorchotrema. The two species Monorchotrema taihokui and M. taichui are compared and shown to be identical. Experimental study of the life history of the heterophyid trematodes shows that they develop in various birds and mammals, including the common laboratory animals, and the suggestion is made that all are probably human parasites. Study of the reports of Yokogawa, Ciurea, and Faust and Nishigori, shows that their conclusion, (that the metacercariae of Metagonimus, Apophallus, and Monorchotrema, respectively, when liberated from their cysts bore into the intestinal wall and develop there to sexual maturity), is not well substantiated. Experiments conducted with laboratory animals demonstrate that Cryptocotyle does not have such an intramucosal stage in its life history.