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A number of chemical agents, and their degradation products, have been recognized as having estrogenic activity on animals. On fish they interfere with endogenous estrogen and in many cases mimic its activity, thus increase estrogenic reponse, with elevated levels of vitellogenin. Hence they are called Endocrine Disrupting Chemicals, EDCs. Vitellogenin levels is often used as a biomarker of exposure to estrogenic pollutants. Wastewater from sewage treatment works contains chemicals known as EDCs, and there are evidence of roach (Rutilus rutilus) living in recipients of such wastewater having traits of both gender, i.e. oocytes in male gonads (intersex) on histological examination. This study was done on roach living in Lake Funbosjön Uppland, Sweden. The lake is a recipient of drainage water from a local refuse dumping area. We also investigated a group of roach that had been kept in laboratory environment for 4-5 years. They were captured in Ekoln, a part of Lake Mälaren. The results of the study showed only one intersex individual from Lake Funbosjön out of 39 males. The laboratory roach had a higher incidense of intersex; 7 out of 26 males had oocytes in testis. The conclusion of the study is that intersexuality on roach is a complex phenomenon, possibly induced by natural variation and environmental factors. The material in the study is relatively small, and further investigations need to be done.