A PWR nuclear power plant at Tihange (Belgium) causes a 2-3 °C temperature in¬crease of the river Meuse. The impact on primary production, measured by 14C_bi_ carbonate incorporation, and on bacterial production, measured by tritiated thymidine incorporation, has been studied. For the latter, an increase of activity from upstream to downstream the power plant has been observed; a similar increase occurred for primary production during a large part of the year, but during summer and sometimes in autumn an important inhibition of photosynthesis is observed. The relative role of direct thermal effects, indirect thermal or non-thermal effects in explaining these observations is discussed. ln the summer the impact of the power plant is likely to significantly affect the oxygen budget of the river. ; info:eu-repo/semantics/published