Since the early 1990s, endocrine disrupting compounds have been recognized as an important environmental threat. Male fish exposed ro effluent from large, metropolitan municipal wastewater treatment facilities (WWTFs) have developed reproductive abnormalities including ovotestes and elevated levels of plasma vitellogenin (Vtg), a plasma protein typically produced by egg-laying females. In the summer of 2000, gonads and plasma Vtg concentrations were examined in feral male fathead minnows (Pimephales promelas) collected from lagoons of 11 small, rural municipal WWTFs and a reference site (a national wildlife refuge) in Iowa. Fathead minnows were captured in traps from five of the 33 lagoons (three per WWTF) sampled. No other fish species were captured. The five lagoons with fathead minnows were found at three WWTFs. Gonad histology indicated only one of 65 (1.5%) male fish living in the lagoons had ovotestes, which was similar to the incidence at a reference site (1 of 29, 3.4%). Plasma Vtg, however, was substantially higher in fish from four of the five lagoons than in fish from the reference site, indicating that fish in lagoons were exposed to estrogenic substances.