Fish stocks in seven Czech, four British and three Dutch reservoirs were surveyed quantitatively once or several times, in a total of 21 surveys. The open water of the reservoirs was studied by mobile survey with scientific echosounders using both vertical and horizontal beaming. The littoral regions were sampled by means of quantitative shore seining carried out on the same night as the acoustic survey. The estimated biomass of fish older than 0+ for different reservoirs ranged from 5 - 600 kg/ha. This wide range is a likely result of site-specific conditions. Although the trophy of the studied reservoirs spans over a wide range of total phosphorus concentrations (30 - 1000 mg.m(-3)), there is no link between the fish biomass and reservoir trophy. Fish abundance and biomass were very low in the basin-like bankside reservoirs whose sides are covered with concrete slabs or asphalt. Two other factors can lead to low fish stocks, namely hydropeaking power generation and long-term biomanipulation, although to a lesser extent. Reduction of the fish stock to