The history of fish, zooplankton and phytoplankton in Rimov Reservoir during its 14 years existence is documented. After the filling of the reservoir in autumn 1978, the biomass of planktivorous fish increased from near zero to 650 kg ha(-1) by the fourth year of the reservoir’s existence, after which the fish stock continually decreased under various forms of biomanipulative pressure, down to a value of about 100 kg ha(-1). This development of fish stock was accompanied by changes in zooplankton size structure, but a cascading effect down to reduction of phytoplankton biomass was not pronounced. The changes of zooplankton body size structure were: increasing or decreasing average size of Daphnia galeata (the main cladoceran herbivore species) and changes in relative proportion of cladoceran species, but none of the main zooplankton species was replaced or disappeared. The variation in phytoplankton biomass was explained by variation in phosphorus concentration and not related to zooplankton structure. It was concluded that the critical biomass of planktivorous fish below which the effect of lowering phytoplankton biomass could be observed lies below 100 kg of planktivorous fish per hectare. An attempt is made to quantify the relationship between fish stock density and the size structure of zooplankton potentially able to influence top-down effects on phytoplankton.