For the three London reservoirs studied, there is new information on the impact of fish populations on their ecology and management, In 1993, the fish stocks were studied quantitatively by night inshore seining and open water dual-beam sonar. Fish biomass was low in two reservoirs (24.2 kg/ha in Wraysbury and 42.2 kg/ha in Queen Elizabeth II) but 3-5 times higher in Queen Mary (both in 1993, 124.3 kg/ha, and in 1994, 145.0 kg/ha); these differences were largely due to the possibilities for cyprinid reproduction in QMR. The heavier fish-predation pressure in QMR is;associated with a small-bodied zooplankton less capable of controlling phytoplankton growths, The management implications are whether the worsening water quality in QMR can be reversed by biomanipulative measures - reduction of cyprinids and/or addition of piscivores.