Triglochin maritima L. was found in 1987 in the Rytisuo fen, Oulanka National Park, Kuusamo, NE Finland. This find is near (about 3.5 km from) the only hitherto known inland locality of the species in Finland, Purkuputaansuo and its vicinity. The small Rytisuo population grows in the edge of the eutrophic pine mire, on thin peat (ca. 0.5 m). It is presumably of recent origin, perhaps having immigrated from the larger Purkuputaansuo population, where Triglochin maritima has grown for at least 8000 years. Vegetation and ecology of Triglochin in both sites are briefly compared.