The present paper describes an inland occurrence of Triglochin maritima L. discovered in northern Kuusamo, in northeast Finland. The species grows over a large area of a fen which is exceptionally rich in electrolytes, and resembles in that respect the coastal sites of the plant. The most abundant electrolyte in the Kuusamo fen, however, is calcium, not sodium or chloride as on the coast. The differences between the coastal and inland growing sites can also be seen in the chemical analyses of the plants. Thus it can be maintained that the species requires a high electrolyte content, but not necessarily the presence of sodium chloride.The origin of the occurrence in Kuusamo goes back to the Preboreal period, from whence onwards it has remained in isolation from the coastal populations. The inland occurrence further illuminates certain aspects in the history of the Primula sibirica group of species.