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The paper gives data on an opisthorchiasis focus in the Taishetsky district, Irkutsk Region. The species affiliation of Bithynia from the Biryusa River basin was established. These mollusks are morphologically close to Opisthorchophorus hispanica (= Bithynia inflata). No O. felineus trematodes were found in the snails examined. Roach, dace, and bream were the second intermediate hosts. Analysis of long-term data revealed that fish and human infection rates have remained comparatively high for almost 20 years. The peak for Opisthorchis infestation in carps were in 1982 (9.3%), 1983 (2.9%), 1998 (5.5%), 2000 (2.5%), and 2007 (5.9%). In other years, fish infection ranged from 0.4 to 1.6%. The performed investigations showed that dace and bream were most infected. Data on the focus area that was 1500 km2 are first presented. The Irkutsk focus of opisthorchiasis is considered to be little studied since there is no evidence (infection rate, prevalence) for mollusks, wild (muskrat, otter, bank vole, fox, wolf) and domestic (dog, pig) animals. The considerable pathogenicity of the parasite and the natural focal pattern of the disease call for regular investigations at all stages of its life cycle: mollusks, fishes, fish-eating mammals, local residents, as well as for comprehensive analysis and elaboration of preventive and controlling measures.