Technical Abstract: The presence of endocrine active compounds such as estrogens in treated wastewater effluent and their effects on aquatic life are causing concern among aquatic resource managers. In contrast to 17B-estradiol (E2), the steroid hormone produced by all vertebrates, the biological effects of estrone (E1), one of its breakdown products are less understood, even though the aquatic concentrations of E1 are often higher than those of E2. Our goal was determine if exposure of fathead minnows to environmentally relevant concentrations of E1 could result in similar biological effects found with exposure to E2, and if reproduction was negatively impacted. In two replicate experiments, we exposed mature fathead minnows to three concentrations of each estrogen for 21 days in a flowthrough exposure system and measured a broad suite of anatomical (body indices, histopathology), physiological (plasma vitellogenin), behavioral (nest defense), and reproductive (fecundity, fertility, hatching) endpoints. These endpoints have previously been associated with adverse effects of estrogenic exposures. While body length and weight parameters were unaltered by exposure, secondary sex characteristics exhibited an exposure concentrated-related decline in male fathead minnows. Interestingly, low concentrations of estrone (~15ng L-1) enhanced the aggressiveness of male fathead minnows in a behavioral assay. Vitellogenin concentrations in male fish increased with higher concentrations of both estrogens, but remained unchanged in all female treatments. A decrease in fecundity was observed at high concentrations of E2 as compared with control minnows. These results suggest that E1, at concentrations previously found in waters receiving wastewater effluent, can have biological effects on fish.