AimsThe aims of this study were to investigate the invitro antibacterial activity of the essential oils (EOs) of Hesperozygis ringens (HREO), popularly known as espanta-pulga' and two different species of basil, Ocimum gratissimum (OGEO) and Ocimum americanum (OAEO), as well as, the potential of these products to be used in silver catfish (Rhamdia quelen) infected with Aeromonas hydrophila. Methods and ResultsOGEO and HREO showed better antibacterial activity invitro. Subinhibitory concentrations of all EOs inhibited haemolysis caused by Aer.hydrophila in fish erythrocytes (100% reduction for OAEO at 100gml(-1) and more than 90% for HREO and OGEO at 150gml(-1)). However, OAEO and HREO showed the best survival results (75 and 70% respectively) after their use as treatment (therapeutic baths1h daily/5days) in silver catfish experimentally infected with Aer.hydrophila. A second invivo assay using healthy fish was conducted to verify the potential of the EOs (preventive baths1h daily/5days) to promote fish survival. Fish exposed to HREO and OAEO and their diluent (ethanol) showed significant lower haematocrit values and higher complement system activity compared to control. Plasma cortisol level was significantly higher in the groups exposed to both EOs. There was no significant difference in survival of silver catfish challenged with Aer.hydrophila after preventive baths with HREO, OAEO and control group. ConclusionsAll tested EOs showed invitro antibacterial properties against Aer.hydrophila and HREO and OAEO showed potential to be used in the treatment of infected fish. Significance and Impact of the StudyThese products can be used in aquaculture as therapeutic and prophylactic agents against fish pathogens, with antimicrobial and/or immunostimulant properties.