The London reservoirs sited in the lower Thames valley form part of a continuously flowing, drinking water supply system and as such have been wholly designed, constructed and operated by man for this sole function. This paper adds some information on the potential impact of the fish populations on the ecology of these relatively deep reservoirs. The fish fauna was studied by night shore seining (to detect inshore fish communities) and acoustically (to detect the offshore fish communities). Ruffe (Gymnocephalus cernuus) and perch (Perca fluviatilis) are the main species capable of reproduction on the steeply sloping concrete walls of the reservoirs. Cyprinids are almost absent in Wraysbury Reservoir whilst in Queen Mary and Queen Elizabeth I1 reservoirs they are more abundantly represented due to enhanced spawning possibilities associated with inundated marginal terrestrial plants in Queen Mary and the net-sides ofempty fish cages in Queen Elizabeth I1 reservoir. Fish biomasses of the three London reservoirs studied are low: 6.8 kg/ha in Wraysbury Reservoir, 28.6 kg/ha in Queen Mary reservoir and 45.6 kg/ha in Queen Elizabeth I1 Reservoir. Coinciding with this is a zooplankton of unusually large-sized cladocerans, largely daphnids, and high fish growth rates.