Aims This study investigated the in vitro antibacterial activity of Hesperozygis ringens (Benth.) Epling leaf extracts against fish pathogenic bacteria, as well as the in vivo activity of the most active extract in silver catfish (Rhamdia quelen) experimentally infected with Aeromonas hydrophila. Moreover, the chemical composition of the extract used in the survival assay was evaluated. Methods and Results Only hexane extract (HEHR) showed in vitro antibacterial activity (MIC and MBC ranging from 1600 to 3200 mu g ml(-1)) against clinical isolates of A. hydrophila, Raoultella ornithinolytica and Citrobacter freundii, obtained from naturally infected silver catfish, and A. hydrophilaATCC 7966. The major compound of the volatile fraction of HEHR was determined as pulegone. HEHR promoted a 93 center dot 33% relative survival rate of silver catfish experimentally infected with A. hydrophila 7 days after a single therapeutic bath at 30 mg l(-1), while florfenicol at 4 mg l(-1), which promoted a 60% relative survival rate. Conclusions The antibacterial activity of H. ringens (Benth.) Epling leaf extracts seems to be related to phytochemicals of apolar character, since HEHR promoted better survival rate of infected animals than florfenicol. Significance and Impact of the Study The HEHR has potential to be used in the control and treatment of bacterial infections in organic aquaculture.