[en] The object of our study was to determine the thermal impact of a nuclear power plant, Pressured Water Reactor (PWR), on fish living in the river Meuse (Belgium). This paper describes and compares the species composition of fish populations as well as the reproductive cycle of the coarse fish Rutilus rutilus L from two sampling sites, a few kilometers upstream and downstream of the heated discharges. The investigations were carried out from April 1979 to October 1983. Perceptible heating (2-3⁰ C) didn't apparently affect from a qualitative point of view the fish fauna of the river Meuse. So far, 23 species have been identified in both areas by monthly catches (electrofishing and gill nets). Examination of the reproductive cycle of the roach, based in the evolution of the gonado-somatic relationship (GSR) and daily observations on the field did not real acceleration of the cycle nor a noticeable advance of the spawning period. Spawning occurred around mid-May when the water temperature reached 15⁰ C, a few days earlier than at the upper sampling site. In 1983 however, a few eggs were seen downstream of the heated discharge one month earlier. Furthermore, the ovary weight seemed to be somewhat higher downstream at sampling dates during December. Further investigations are fecundity of the roach carried out in 1979 and 1980 showed that this phenomenon corresponded more to an earlier development and to a greater proportion of large ripening eggs that to a greater number of eggs. However, final size of eggs just after spawning is closely the same (1.7 mm) in both stations