ABSTRACT: We investigated the effects of cell-free filtrates of the cyanobacteria Nodularia spumigena, Aphanizomenon sp. and Anabaena sp. on a brackish-water plankton community (< 150 µm). In a laboratory experiment, wemonitored the concentration of chlorophyll a and cell numbers of bacteria, phytoplankton and ciliates in the experimental units treated with cyanobacterial filtrates and the control medium. Cyanobacterial filtrates altered the cell numbers of manyphytoplankton groups; they stimulated both colonial (Snowella spp.) and filamentous cyanobacteria (Pseudanabaena spp., Anabaena spp., Aphanizomenon sp., N. spumigena), a chlorophyte (Oocystis sp.), adinoflagellate (Amphidinium sp.) and nanoflagellates, but inhibited cryptophytes. The filtrates also increased the numbers of bacteria at the beginning of the experiment, whereas they had no effect on ciliate abundances. Our results suggest thatcyanobacteria may enhance the effects of eutrophication by stimulating various phytoplankton groups. The specific compounds causing this stimulation remain to be studied. On the other hand, cyanobacteria also seem able to reduce the biomass of certainphytoplankton species, via an allelopathic mechanism.