The synthetic estrogen, 17alpha-ethinylestradiol (EE(2)), is the active component in oral contraceptive pills. It is excreted from the human body in high amounts and released via sewage treatment plant effluent into aquatic environments. In fish, estrogen receptors have strong binding affinities for EE(2), and exposure raises the possibility of adverse neuroendocrine responses in aquatic animals. In the present study we explored the effects of dissolved-phase EE(2) on the dynamics of male-male aggression and courtship behaviors in adult zebrafish. Further, we assessed whether the behavioral effects of EE(2) result in changes in male offspring paternity. We scored the aggressive behaviors of individual unexposed males and categorized these fish as either dominant or subordinate. We then exposed dominant males to EE(2) at doses of 0, 0.5, 5.0, and 50.0ng/L for 48h. Subsequent trials examined the agonistic behaviors of males in two testing scenarios: (1) a dyadic encounter with another male alone, and (2) a competitive spawning interaction with another male and three adult females. Competitive spawning tests were also used to assess the impacts of EE(2) exposure on courtship behavior and paternity using males that were homozygous for green fluorescent protein (GFP) expression under the control of the islet-1 promoter. We found that EE(2) at all exposure concentrations reduced male aggression during male-male dyadic encounters and caused a social dominance reversal in 50% of the fish at the highest exposure dose (50ng/L EE(2)). The frequency of courtship-specific behavior decreased in dominant males exposed to the steroid, though this effect was only significant for the lowest dose group (0.5ng/L EE(2)). In the highest exposure group (50ng/L EE(2)), 50% of dominant males relinquished paternal dominance. Our results show that short-term exposure to EE(2) at environmentally relevant levels can alter aggression, and shift individual social status and reproductive success in male zebrafish.