Diethylstilbestrol (DES) is a synthetic estrogen that has been banned for use in humans, but still is employed in livestock and aquaculture operations in some parts of the world. Detectable concentrations of DES in effluent and surface waters have been reported to range from slightly below 1 to greater than 10 ng/L. Little is known, however, concerning the toxicological potency of DES in fish. In this study, sexually mature fathead minnows (Pimephales promelas) of both sexes were exposed to 1, 10, or 100 ng of DES/L of water in a flow-through system. Tissue concentrations of DES and changes in a number of estrogen-responsive end points were measured in the fish at the end of a 4 d exposure and after a 4 d depuration/recovery period in clean water. Accumulation of DES was sex-dependent, with females exhibiting higher tissue residues than males after the 4 d exposure. The observed bioconcentration of DES in the fish was about 1 order of magnitude lower than that predicted on the basis of the octanol-water partition coefficient of the chemical, suggesting relatively efficient metabolic clearance by the fish. Exposure to 1, 10, or 100 ng of DES/L caused decreased testis weight and morphological demasculinization of males (regression of dorsal nuptial tubercles). Diethylstilbesterol induced plasma vitellogenin (VTG) in both sexes at water concentrations ≥10 ng/L; this response (especially in males) persisted through the end of the 4 d recovery period. Hepatic transcripts of VTG and estrogen receptor-α also were affected at DES concentrations ≥10 ng/L. Evaluation of transcript profiles in the liver of females using a 15K-gene fathead minnow microarray revealed a concentration-dependent change in gene expression, with mostly up-regulated transcripts after the exposure and substantial numbers of down-regulated gene products after depuration. Genes previously identified as vitellogenesis-related and regulated by 17β-estradiol were significantly enriched among those differentially expressed following exposure to DES. Overall, our studies show that DES causes a range of responses in fish at water concentrations comparable to those reported in the environment and that in vivo potency of the estrogen is on par with that of the better-studied estrogenic contaminant 17α-ethinylestradiol.