Gonadal intersex and high prevalences of the female phenotype have been observed in fish populations in urbanized areas. Environmental estrogens discharged in sewage treatment plant effluents may be responsible for feminization of fish but many compounds with the potential to induce these responses occur in effluents, including natural and synthetic estrogen hormones, degradation products of alkylphenol ethoxylate surfactants, and plasticizers. In this study, the estrogen hormones 17 alpha-ethinylestradiol, 17 beta-estradiol, estrone, and estriol induced intersex (i.e., testis-ova) and altered sex in Japanese medaka (Oryzias latipes) when these fish were exposed to nanogram per liter concentrations of test compounds from hatch to approximately 100 d after hatch. A mix of nonylphenol mono- and diethoxylate induced a weak response and a mix of nonylphenol mono- and diethoxycarboxylate did not give a response in this assay at microgram per liter concentrations, indicating that these degradation products of nonylphenol ethoxylates have little or no estrogenic activity in fish. Bisphenol A induced testis-ova in medaka exposed to a concentration of 10 micrograms/L, but diethylhexyl phthalate did not induce a response. Results with the medaka assay were consistent with estrogenic responses in the yeast estrogen screening assay. Analyses of monitoring data reported in the literature indicate that concentrations of estrogen hormones detected in the final effluents of sewage treatment plants are generally greater than the lowest-observed-effect levels for alterations to gonadal development in medaka.