The thermal pollution of the Tihange Nuclear Power Plant increases the temperature (± 3 C) of the river Meuse and has precise effects on the reproductive cycle of the roach (Rutilus rutilus) downstream: welldefined, ovarial development, acceleration of maturation, and earlier laying of eggs (3 weeks). The volume of the different testicular cell types appears to be more important in males downstream. Spermatozoa are retained for a longer period in both populations (upstream and downstream) as a result of diminished spawning-grounds along the river.