The object of the study is the whitefish Coregonus lavaretus (Linnaeus, 1758), which inhabits the Nizhnetulomskoe Reservoir (NTV) in the Tuloma River catchment. Parasitological analysis of the whitefish sampled in the framework of annual ichthyological monitoring was carried out. In two fish specimens, many large cysts (up to 7.0 mm) containing plerocercoids of the parasite Triaenophorus nodulosus (Pallas, 1781) were found on the fish liver. In waters of the Murmansk Region, this was the first case of whitefish infestationwith this helminth species. The infection rate was 5.4 %. In recent years, the rainbow trout Oncorhynchus mykiss (Walbaum, 1792) has been farmed in the NTV waters. To cagereared fish, this cestode is a pathogenic and highly dangerous parasite. The paper considers the vulnerability of the reservoir’s ecosystem to the impact, and the manifestation of fish diseases of various etiologies in recent years. In particular, there are occurrences of a disease of unclear etiology, ulcerative skin necrosis, in anadromous Atlantic salmon migrants, as well as infestation of juvenile salmon with the monogenean Gyrodactylys salaris (Malmberg, 1957). In the light of the alarming epizootic situation and due to the finding of a helminth not typical in whitefish in the NTV, integrated ichthyological, parasitological and hydrobiological studies are needed. The aim of these studies is to determine the extent and dynamics of fish infestation with the cestode T. nodulosus, and to track the transmission of the infection along the food chain of aquatic organisms in the ongoing succession processes in the ecosystem.