Endocrine disruption and the feminization of male barbel Barbus barbus was examined during a 4-year investigation in the River Jihlava (Danube basin, Czech Republic) that consisted of temporal monitoring of sexual status in 993 tagged barbel and histological verification of sex change in 149 dissected specimens. In 343 recaptured barbel, six specimens had altered (protandric) gamete production, which is a rather exceptional phenomenon (2.8% of the total number of recaptured original tagged males) and does not explain the unbalanced (biased) sexual and size structure of barbel populations that is often observed. Histological gonadal examination revealed intersexuality to be sporadic (2.0%) and more frequent (14.8%) in river stretches of relatively lower and greater organic pollution loads, respectively. The screening of vitellogenin (Vtg) in blood plasma revealed elevated, strongly fluctuating concentrations in males, unrelated to fish size or age, which reached a median of 0.190 μg ml−1 in the moderately polluted stretches, whereas a significantly higher value of 0.732 μg ml−1 was observed in strongly polluted stream stretches. These levels were apparently a result of elevated levels of ambient xenoestrogens.