Of over 6,000 examinees in northeast Thailand, 79% were found to be infected with the human hepatic trematode Opisthorchis viverrini. Some 90% of all examinees over the age of 10 were positive, and it is estimated that over 3.5 million persons in that country harbor the parasite. The snail hosts are Bithynia goniomphalus, B. funiculata, and B. laevis. The oculate and pleurocercous cercaria resembles that of 0. felineus. The flame cell patter of the cercaria as it leaves the snail host is 2[(3 + 3) + (3 + 3 + 3)], differentiating it from the cercaria of 0. felineus, and establishing 0. viverrini and 0. felineus as separate species. The most important fish intermediate hosts are Punteus orphoides, Hampala dispar, and Cyclocheilicthys siaja, although a total of nine species were found to harbor the metacercariae. It is postulated, on the basis of seasonal fluctuations, that the largest number of human infections occur during the last portion of the rainy season and the first third of the dry (September to February). The characteristics which have been reported to differentiate adult 0. viverrini from 0. felineus have been studied but it has not been possible to separate the two species on the morphological characteristics of egg or adult. Thus, although not of practical laboratory significance, at present the only certain means of distinguishing between the two species is the flame cell patterns of cercariae or metacercariae. Little was known of human hepatic trematodes in Thailand or the neighboring countries until 1953-1955 when Sadun (1955) reported that Opisthorchis viverrini was widespread in the northeast. Prior to his investigations only a few scattered reports concerning Opisthorchis in Thailand were available. In 1908 Verdun and Bruyant noted that "Opisthorchis felineus" was found in Indochina, and in 1911 Leiper recovered "Opisthorchis viverrini" from a human autopsy in Chiengmai (northwest) Thailand. Sixteen years later Prommas (1927) reported a single case of "0. felineus" from a Received for publication 22 May 1964. * U. S. Army SEATO Medical Research Laboratory, APO 146, San Francisco, Calif. (Rajavithi Road, Bangkok). t The Bangkok School of Tropical Medicine. This work was supported in part by the U. S. Army Medical R & D Grant No. DA-MD-49193-63-G96. Portions of this work were presented at the 7th International Congress of Tropical Medicine and Malaria, Rio de Janeiro, 1963, and the First International Congress of Parasitology, Rome, 1964. Please direct your request for reprints to: Department of Medical Zoology, Walter Reed Army Institute of Research, Washington, D. C. 20012. human autopsy in northern Thailand, and in 1929 Bedier and Chesneau reported that "0. viverrini" was present in 23% of 1,231 examinees in Takhek, and in 15% of 523 persons in Vientiane, Laotian cities immediately across t e Mekhong River from Thailand. Inasmuch as Sadun (1955) reported only 0. viverrini from Thailand, it appears that the reports of "0. felineus" may have been in error. In any case, following the initial description of O. viverrini by Poirier (1886), uncertainty has surrounded the validity of the species. Erhardt (1935), on the basis of findings of other investigators, but without examining 0. viverrini himself, reported that this species is not identical with 0. tenuicollis, in contrast to the opinions of Price (1932), Vogel (1932), Faust (1949), and Dawes (1946). It should be noted that 0. tenuicollis was recovered from a sea mammal, leading one to believe that marine intermediate hosts were involved. Ejsmont (1937) explained this by stating that these mammals often enter estuaries to feed on cyprinoid fish infected with Opisthorchis metacercariae. He concluded that 0. tenuicollis and 0. felineus should be considered as subspecies. Watson (1960) reported that 0. tenuicollis is synonymous with 0. felineus, thus possibly