The detection of vitellogenin, a yolk precursor protein, may serve as a biomarker for exposure to environmental oestrogens as its induction by xenobiotic oestrogens in the immature and male fish has been reported repeatedly. In the present work, juvenile brown trout were injected with oestradiol (5 μg g−1 body weight oestradiol benzoate) in order to assess the induction and organ distribution of vitellogenin by means of immunohistochemistry. In addition, brown trout collected from Swiss rivers were analysed. Vitellogenin was detected in the oestradiol-injected juvenile trout but not in uninjected controls. The presence of vitellogenin was also demonstrated in a male and an immature feral brown trout from one of two locations downstream of three sewage treatment plants. In contrast, no positive staining was found in livers of trout upstream of the respective plants. The results demonstrate the suitability of immunohistochemistry for monitoring feral fish fo r the presence of vitellogenin production.