Bacterial pathogens cause high fish mor talities and in turn economic losses in fish farms. Inno vative strategies should be applied to control bacterial infections instead of antibiotics to avoid the resistance problem. Consequently, the present investigation stud ied the curative potential of Azadirachta indica leave ethanolic extract (AILEE) on Aeromonas veronii infec tion in Oreochromis niloticus. A preliminary trial was assessed to evaluate the curative dose of AILEE which was found to be 2.5 mg/L. One hundred and sixty fish were divided into equal four groups in four replications,where group 1 and group 2 were non-challenged and treated with 0- and 2.5-mg/L AILEE, respectively. Group 3 and group 4 were challenged with A. vero nii and treated with 0- and 2.5-mg/L AILEE, respec tively for 10 days. A. veronii infection produced severe clinical manifestations and a high mortality rate in the infected fish. Furthermore, the infected fish exhibited a significant rise in the hepatorenal indices (aspartate aminotransferase, alanine aminotransferase, and cre atinine), the oxidant biomarker (malondialdehyde), and the stress indicators (glucose and cortisol). A sig nificant reduction in the protein profile and antioxidant/ immune parameters (catalase, immunoglobulin M, lysozyme, nitric oxide, and phagocytic activity) was observed in the infected fish. Water application of the infected group to 2.5-mg/L AILEE notably ameliorated the hepatorenal indices, the oxidant biomarker, and the stress indicators. Furthermore, AILEE improved the antioxidant/immune indices. Water application of 2.5 mg/L AILEE could be useful against A. veronii infec tion in O. niloticus culture.