Opisthorchiasis is produced by two important species of Opisthorchis, namely, O. felineus (Rivolta, 1884) Blanchard, 1895 and O. viverrini (Poirier, 1886) Stiles and Hassall, 1896, both of which are cat liver flukes but which also infect certain other mammals and humans. O. felineus infections have been found in southern, central, and eastern Europe, Siberia, East Prussia, Poland, northern Korea, and probably also in Japan, India, and the Philippines, but not in the same localities where Clonorchis sinensis has been reported. O. viverrini is an important source of human infection in northeastern Thailand, Laos, and other parts of southeast Asia.