Subfossil remains of a new species of Cladocera (water fleas) of the family Chydoridae in Finland, Alona werestschagini Sinev, were found in the sediments of four lakes above the treeline in northernmost Finnish Lapland. The remains were found in surface sediments of three lakes and in early Holocene sediments of one lake where the species was a pioneer which soon disappeared. The remains of A. werestschagini, except the male postabdomen, closely resemble Alona guttata. In Eurasia A. werestschagini has a wide but patchy distribution in cold climates, suggesting that it is a postglacial relict adapted to cold climate and oligotrophic lakes. Recently it has been found also in Norway and Kola Peninsula. The early Holocene finds indicate that the species spread to northernmost Finland after the retreat of the Scandinavian Ice Sheet. Since the species has been found in lakes in very severe conditions it may be used as a palaeolimnological indicator in sediment studies.