Abstract: The present dissertation describes the morphology and structure of both adult and larval stages of three species of heterophyid trematodes, namely, Centrocestus formosanus Nishigori, 1924; Haplorchis pumilio Looss, 1899 and Procerovum varium Onji Ni-shio, 1916, and Metorchis orientalis Tanabe, 1919, with a view to study the phylogene tic relationship of the families Opisthorochidae and Heterophyidae. The principal features of both families were compared, special emphasis was laid on the comparison of developmental stages and adults.While the development of three heterophyid trematodes is reinvestigated under local conditions, the completion of the life history of Metorchis orientalis is here reported for the first time.Basing on the informations obtained from the life history studies of the four species of trematodes and considering their similarities and dissimilarities in morphological characters, the present investigation came to the conclusion that there exists an intimate relationship between the two families. They should belong to the same superfamily, Opisthorchioidea (Faust, 1929) Vogel 1934.