Fish abundance and size composition in a 28 km stretch of the Vltava River in the Czech capital of Prague were studied using horizontal dual-beam scientific sonar. Very low fish densities (0.6-0.7 fish/100 m(3)) were found above Prague where the river is influenced by a cascade of reservoirs. These have a retention time of more than 100 days and release hypolimnetic water. A significant increase in fish density was found in the Berounka River confluence (a tributary with a normal, seasonal temperature pattern) just above the city (about 10 fish/100 m(3)). The fish stock is fairly low in the city itself because of channelization of the river bed (1.3-3.5 fish/100 m(3)), but much greater densities (14.9-16.9 fish/100 m(3)) were found below the city where the river receives a high organic load of partially treated sewage and has a semi-natural shoreline. The largest average fish size and biomass were also reported from this section.