The high incidence of intersex roach (Rutilus rutilus) in some United Kingdom rivers that has been associated with exposure to sewage treatment works (STWs) effluent led us to hypothesize that top predator fish also may be affected by estrogenic chemicals, because they are likely to bioaccumulate lipophilic compounds through a predator-prey relationship. To investigate this possibility, pike (Esox lucius) were sampled both upstream and downstream of STWs and then examined for total estrogenic activity of their bile, as measured using a yeast-based estrogen assay to determine the degree of recent exposure of the pike to estrogens and vitellogenin induction, and for possible disruption of sexual development, as measured using histological analysis of the gonads. No evidence of severe disruption was found in the sampled fish, which came from 16 sampling sites that were representative of English rivers. However, 14% of pike were intersex, of which 15 of 16 showed patches of male germ cells among predominantly female gonadal tissue. The incidence of masculinization was independent of whether the pike had been sampled upstream or downstream of STWs. Although pike are gonochoristic, it is not known if this masculinization of presumptive female pike is normal or, instead, indicative of endocrine disruption. Vitellogenin concentrations were not elevated in male pike at sites either upstream or downstream of STWs. The results suggest that sexual disruption is not common in pike, a fish at the top of the food chain in the freshwaters of England.