The aim of this study was to assess the health indicators and reproductive endpoints in a wild population of lambari Astyanax fasciatus, a small characid fish widely distributed in South America. A range of biomarkers, from the molecular to population levels, was evaluated in adult fish sampled from five sites from the Furnas Reservoir, Grande River. At the sites that receive untreated agriculture and industrial residues, fish showed reduced body size and delayed gonadal maturation. Fish from the sites located immediately downstream from the municipal wastewater discharges exhibited feminisation, intersex and evidence of contamination by xenoestrogens. Elevated levels of zona radiata proteins were found by Western blot in the livers of male fish from three contaminated sites. Overall, this study provides the first evidence of endocrine disruption in a wild fish population inhabiting the Furnas Reservoir, and suggests water contamination by oestrogenic compounds and agricultural residues with a possible risk to human health and wildlife.