Aggressive behaviors are widespread among animals and are critical in the competition for resources. The physiological mechanisms underlying aggression have mostly been examined in breeding males, in which gonadal androgens, acting in part through their aromatization to estrogens, have a key role. There are two alternative models that contribute to further understanding hormonal mechanisms underlying aggression: aggression displayed in the non-breeding season, when gonadal steroids are low, and female aggression. In this study we approach, for the first time, the modulatory role of estrogens and androgens upon non-breeding aggression in a wild female teleost fish. We characterized female aggression in the weakly electric fish Gymnotus omarorum and carried out acute treatments 1 h prior to agonistic encounters in dyads treated with either an aromatase inhibitor or an antagonist of androgen receptors. Anti-androgen treatment had no effect on behavior whereas acute aromatase inhibition caused a strong distortion of aggressive behavior. Territorial non-breeding aggression was robust and depended on rapid estrogen actions to maintain high levels of aggression, and ultimately reach conflict resolution from which dominant/subordinate status emerged. Our results, taken together with our own reports in males and the contributions from non-breeding aggression in bird and mammal models, suggest a common strategy involving fast-acting estrogens in the control of this behavior across species. In addition, further analysis of female non-breeding aggression may shed light on potential sexual differences in the fine tuning of social behaviors.