The Desmidflora of the Mosterdveen (a complex of small fen soaks in the Netherlands province of Gelderland) provides a good example of a microphyte cenose characteristic of an oligotrophic type of water (as described by BEIJERINCK, 1926), which has in the meantime become very rare. The most typical indicator species found in the Mosterdveen are Cosmarium ralfsii Bréb., Micrasterias jenneri Ralfs, Euastrum crassum (Bréb.) Kütz., and Xanthidium armatum (Bréb.) Rabenh. Some species whose occurrence and distribution in the Netherlands is not so well known, partly owing to their problematic taxonomy, are discussed separately.