We conducted a 4 mo mesocosm experiment to elucidate the cascading impact of the zooplanktivorous three-spined stickleback Gasterosteus aculeatus on phytoplankton in slightly brackish lakes. The mesocosms were nutrient-enriched with low salinity (2 psu) and contrasting densities of sticklebacks (0 to 10 fish m -2 ). Total phytoplankton biovolume increased by 2 orders of magnitude when stickleback density increased from 0 to 3 to 6 to 10 fish m -2 , most likely reflecting a parallel change in zooplankton biomass and size distribution. Phytoplankton size distribution was affected in that large forms dominated at high fish density, which most likely reflects a parallel reduc- tion in size of zooplankton. A few taxa (especially Cryptophyceae) dominated at fish densities above 3 to 6 fish m -2 , whereas higher diversity (characterised by taxa of Bacillariophyceae and Chloro- phyceae) was recorded at fish densities below 3 to 6 m -2 . However, genera richness showed no sig- nificant relationship with fish density. The threshold fish density for a transition from low to high phytoplankton biovolume at these high nutrient levels was 3 to 6 fish m -2 . This level lies well below stickleback densities found in many eutrophic, low-salinity lagoons, implying that the intensity of the cascading top-down effect exerted by planktivorous fish on the phytoplankton abundance and com- munity structure is of importance.