The incidence of intersex was examined in freshwater fish in one English and three French rivers. Over 331 fish, mainly roach, were sampled above and below the major sewage treatment works (STW) on each river and the incidence of intersex was recorded by histological examination of fish gonads. No intersex conditions were observed in male fish collected either above or below the major STW on the River Ouse in Sussex. In contrast, roach showing primary oocytes in their testes were found in all three French rivers with the incidence ranging from 9 to 21 % of the males. An intersex chubb and gudgeon were also caught, showing that other species can suffer from this pathology. In addition, roach in the River Seine were heavily parasitised with the tapeworm Ligula intestinalis which may impair the reproductive capabilities of the fish.