Little information exists about the ecological determinants of the mollusc species composition of Scandinavian fens. Therefore, we investigated water chemistry, hydrology, climate and vegetation composition of fens in southern and central Norway and in Sweden hosting mollusc assemblages. According to the results of the MDS ordination, variation in the mollusc composition was related to the hydrology of the studied sites, reflecting a transition from soligenous to topogenous fens. The second axis of the MDS reflected a complex gradient of mineral richness and climatic conditions. By analogy, cluster analysis revealed three main groups of sites, i.e. topogenous fens characterized by a frequent occurrence of Vertigo lilljeborgi, soligenous lowland fens characterized by Cochlicopa lubrica and Nesovitrea hammonis, and soligenous highland fens characterized by Columella columella. Results of the RDA closely resembled those of the MDS, suggesting that the selected explanatory variables defined mollusc assemblages well.