Opisthorchiasis, together with its associated cholangiocarcinoma, is one of the most important human parasit ic diseases on continental Southeast Asia. A great deal of epidemiological data from humans is available on this disease, particularly from the northeast of Thailand, however, only limited infomation is available on those aspects of the life cycde relating toits Bithynia (Gastropoda) and cyprinid fish intemediate hosts. Here we review the information which is available on the Bithynia hosts of Opisthorchis viverrini. Only one major ecological study has been carried out at one site on a single species of Bithynia. Weshow not only that detailed ecological studies are required to darify the epidemiology of opisthorchiasis, but also that the taxonomic status of the Bthynia species transmitting O. viverrini requires clarification.