SUMMARY.1. The abundance of pianktivorous juvenile yellow perch,Perca flavescens, was manipulated in three 750 m3enclosures in a eutrophic lake.2. There was a significant negative relationship between fish and zoopiankton biomasses. At high fish densities the zooplankton community was dominated by small filter‐feeding cladocera. primarily bosmi‐ nids. At low fish densities the zooplankton community was dominated by large filter‐feeding cladocera, primarily daphnids.3. There was no significant relationship between zooplankton and phytoplankton biomasses when considered over the whole experiment but there was a trend towards lower phytoplankton biomass in the enclosure dominated by daphnids during mid‐summer.4. We conclude that although planktivorous fish have a strong negative impact on zooplankton community biomass and size structure, the relationship at the next lower trophic level, zooplankton and phytoplankton, is much weaker. Therefore, the biomanipulation of planktivorous fish populations as a management technique to control phytoplankton abundance is largely inef