Malacofaunal and ichthyoparasitological studies were conducted in the floodplain-river ecosystem in the middle course of the Amur River. The studies covered its channels and tributaries, such as Big and Small Bira, Bidzhan, In, and Vertoprashikha Rivers, in the Birobidzhan, Leninskoye, and Smidovich districts of the Jewish Autonomic Region in the second half of August 2011. It was established that there were biotopes of the mollusk Parafossalurus manchouricus, the first intermediate host of Clonorchis sinensis, in the exosystems of the In, Bira, and Bidzhan Rivers. Sporadic P. manchouricus specimens were found in the Bidzhan channel in the vicinity of its estuary and on the flooded bank of the In River. The bay of the Bira River (the town of Birobidzhan) exhibited a biotope of P. manchouricus with a mollusk population density of 180 to 300 specimens/m2. Seven of 16 examined fish species showed metacercariae of Clonorchis. Among them there were not only Cyprinidae, but also representatives of other families and orders. The two-year-olds of orca-skripuns (Pseudobagrus fulvidraco), orca-whips (Tachysyrus ussuriensis), and Brazhnikov's orsas (Liocassis braschnikowi) were infected with C. sinensis metacercariae. Clonorchis metacercariae were first detected in Amur sleepers (Percottus glehni). This fish species has not been heretofore recorded as the second intermediate host ofC. sinensis. The highest invasion extent for C. sinensis metacercariae was revealed in Amur bitterlings (Rhodeus sericeus) born during the current year. In the biotopes ofdifferent water reservoirs, it changed from 10.0 to 48.0%. In 4 of 6 water reservoirs, Amur bitterlings were found to have larvae of this species of trematodes. The invasion rates were in the range from 1 to 4 metacercariae per fish. Only in individual cases, they were as great as 28 parasitic larvae per fish.