Abstract <p>The current state of the semiaquatic mammalian complex of the Vyatka River floodplain under state protection (in the Nurgush Nature Reserve and its buffer zone in the Kirov oblast) has been studied: the species composition, the population trend, their distribution over the territory, and the environmental impact. The census was taken according to a generally accepted method covering about 50% of the studied territory. It is shown that the Nurgush Nature Reserve has restored and protected a full complex of semiaquatic mammals typical of the European southern Boreal floodplain (seven species). The number of beavers has reached the capacity of habitats and fluctuates slightly around its size. The muskrat introduced in the study area has an extremely low population for the last decade; therefore, the species does not play any role in ecosystems. The native European mink has been completely replaced by the introduced American mink, which has occupied the ecological niche of the displaced species. The otter under conditions of state protection from 2005 to 2024 has turned from a critically low-numbered species into a common one. The otter population recovery seems to be a limiting factor for the muskrat. The Russian desman has been recognized as a species with an extremely low density in the Nurgush Nature Reserve; the probable population size in the reserve and its buffer zone is up to ten individuals. The Eurasian water shrew is a stable component of the mammalian complex of the studied area. Due to its low numbers, this species has no environmental significance. The European water vole was not numerous during the period of our research, making it impossible to fully assess its importance in the communities.</p>