The aquatic environment is the final destination for most chemicals released in the environment, as a consequence freshwater populations are inevitably exposed and may suffer from adverse effects. Fish have been widely used as models for determining the effect of environmental pollutants on vertebrate reproductive function. Several studies have demonstrated endocrine disrupting effects on fish living in rivers that receive treated sewage effluents. To investigate the estrogenic effects of effluents from the Seine River, plasmatic vitellogenin concentrations, a specific female protein, and gonadal histopathologies were studied in roach (n = 723). We have identified a significant increase in plasmatic vitellogenin, as assessed in male fish from contaminated sites, indicating that they are exposed to xenoestrogens. Intersex fish, characterised by oocytes in the testes, represented 14% of the population. Severe intersexuality may decrease the fertility of the individual. The sex ratio was found to be significantly altered (30% of males), indicating feminisation of the roach population in the Seine River.