In the ecosystem of the Belaya River, fishes of the family Cyprinidae were infected with three epidemically important species oftrematodes of the family Opisthorchidae: Opisthorchisfelineus, Metorchis bilis, and Pseudamphistomum truncatumn. No biotopes of mollusks of the genus Codiella were detected. The contamination of underyearlings with-Ofelineus and M.bilis metacercariae indicates that there are cores of opisthorchiasis and metorchiasis (M.bifis) foci in the ecosystem of the lower course of the river. The presence of biotopes of the genus Bithynia mollusks, the first intermediate host.for P. truncatum, in the waters, but the absence of fishes infected with P.truncatum metacercariae among the examined under- yearlings may lead to the conclusion that there are no cores of pseudamphistomiasis foci. Infestation of onein 97 older individual muvarica (Albumus alburnus) with trematode metacercariae is evidence of its infection outside the floodplain river ecosystems of the Belaya River and subsequent migration of the fish into the lowerreaches of the river, where a barren zone of a pseudamphistomiass focus is formed. In this ecosystem there is a risk of human infection with epidemically important species of trematodes of the family Opisthorchidae (O.felineus, M.bifis, and P.truncatum)-when eating the fish caught in the river, its tributaries, and floodplain waterbodies. When identifying the metacercariae and detecting the natural foci of trematodes, it is necessary to take into account the fact that the fishes of the family Cyprinidae in the river ecosystem have four types of Opisthorchis metacercariae (after D.A.Razmashkin).