A survey was made from 1976 to 1981 of the possible effects of Tihange nuclear power plant on the Meuse river. Phytoplankton and periphyton were affected only during low water flow. Both decreases and increases of biomass were recorded for phytoplankton but, for periphyton, only increases. The structure of diatom communities is affected downstream of the plant during low water flow, because sensitive species are replaced by more tolerant ones. In the reach downstream of the plant, the maximum observed temperature increase is 4.2 °C and the maximum decrease in dissolved oxygen is 15%. Except for these two parameters immediately below the plant, any changes observed cannot be attributed solely to the nuclear power plant, but rather to the combination of thermal effects with effluents from other industries which together contribute to the more significant changes observed further downstream.