Knowledge of embryological development and larval stages has been generally applied to the problem of relationships among animals and has given strong evidence of affinities between certain groups. In the digenetic trematodes, that knowledge has revealed hitherto unsuspected affinities and has prompted a re-examination of earlier concepts and taxonomic schemes. As a result, several investigators have proposed, for such restricted groups as families and superfamilies, revised concepts which have been incorporated in the comprehensive taxonomic scheme proposed by La Rue (1957). Because information from life history studies is still lacking for many groups, chiefly those occurring in marine hosts, La Rue's scheme is admittedly tentative in respect to such groups. Among trematodes parasitizing freshwater and terrestrial hosts, however, such knowledge is more extensive, so that at present there are few such trematodes whose affinities remain obscure. One is the turtle lung fluke, Heronimus chelydrae, whose anatomy is so unusual that the species has long been placed in a distinct family, the Heronimidae, with no apparently close relationships to any other group of trematodes. The present study was prompted by the isolated position of that family and by the occurrence of H. chelydrae in several species of turtles in the vicinity of Lafayette, Indiana, thus assuring abundance of material. Heronimus chelydrae was originally described from the lungs of Chelydra serpentina by W. G. MacCallum (1902), who assigned the trematode to Monticelli's family Monostomidae, but recognized its unique differences from other species in that family. Barker and Parsons (1914; 1917) proposed the name Aorchis extensus for a lung fluke from Chrysemys marginata, and believed by them to be distinct from H. chelydrae. To include the genera Heronimus and Aorchis, Ward (1917) erected the family Heronimidae; he later (1918) emended its diagnosis slightly and stated that because H. chelydrae and A. extensus were imperfectly known, they might prove to belong to the same genus, if not indeed to the same species. That possibility was confirmed by Stunkard (1919) who compared lung flukes from six species of turtles and reduced A. extensus to synonymy with H. chelydrae. G. A. MacCallum (1921) tentatively described two new species, H. geomydae from Geomyda punctularia, a Trinidad turtle, and H. maternum from Emys blandingi collected in Ohio. Caballero (1940), however, compared flukes from 16 Kinesternon hirtipes with the type specimens of H. geomydae and H. maternum and concluded that all were identical with H. chelydrae. That species would, thus, be the sole representative of the family Heronimidae.