[in Russian with English abstract: ] New data on the distribution, abundance, structure, and ecology of coenopopulations of Leptogium rivulare in the Kuloy Protected Area (Arkhangelsk Region, Russia) are presented. Until this study, only two records of the species, represented by single thalli, were known from the southeastern part of the region, at a considerable distance from the new locality. The species was recorded in the floodplain of the Kuloy River and its tributaries, the Kelda and Polta rivers, at eight of the 17 surveyed sample plots (47%), within willow forests, paludified herb-rich spruce forests, and a willow sedge-hypnum swamp. All recorded habitats are associated with floodplain communities located along riverbanks or near oxbow lakes, with an average spring flood depth of approximately 2 m. A total of 38 host trees were recorded, supporting 530 thalli with a total area of 527.5 cm2. Thalli predominantly occur on trunks of living trees and, considerably less frequently, on standing dead trees of Salix caprea, as well as S. pyrolifolia and S. dasyclados. In most communities, the number of trees colonized by L. rivulare ranges from one to four, whereas in one phytocoenosis the number of host trees reaches 21. Thalli representing three ontogenetic periods were identified within the coenopopulations: pre-generative (immature and virginal stages), generative (with apothecia), and post-generative (subsenile and senile stages). Generative thalli predominate and are characterized by the largest sizes, while thalli of other ontogenetic stages occur in close proximity to fertile individuals. Overall, the coenopopulations of L. rivulare in the studied communities exhibit a sexual reproduction strategy. The rare occurrence of subsenile and senile thalli suggests that the processes of their death and destruction proceed relatively quickly due to the mechanical impact of the flood processes. The data obtained in the Kuloy Protected Area characterize this system of L. rivulare coenopopulations as one of the largest currently known in Europe. The occurrence of the species within a protected area in minimally disturbed floodplain communities with a stable hydrological regime and abundant suitable substrate promotes the persistence of this rare species. Keywords: Red Data Book of Russian Federation, Kuloy Protected Area, population ecology, river floodplains, nor­thern taiga forests, cyanolichens.