Five populations of the circumpolar Arctic-alpine lichen species Cetraria nivalis were sampled along a latitudinal gradient from the temperate to the Arctic climatic zone. The populations were compared with respect to their CO2 exchange, water relations and morphology/anatomy. Provenances from places along this latitudinal gradient were transplanted in 1983/84 to a site near the Scientific Research Station in Abisko, to see whether they are identical with plastic acclimation ability or whether they represent different ecotypes. The five populations represent two major geographical groups, a ‘southern’ and a ‘northern’ one, with respect to some morphological and anatomical features, chlorophyll content and water relations. The transplanted C. nivalis provenances showed a tendency towards increasing net photosynthetic rates with origin in the higher latitudes. The CO2 exchange response of the native populations was the reverse of this tendency although CO2 exchange was in general flexible. It is concluded that the investigated C. nivalis populations were different ecotypes because of genetic differences and a distinct seasonal plasticity. The ecotypes are interpreted as a result of isolation in different refugia during the Pleistocene.