Clarias batrachus (Linn) is a species of catfish of the family claridae. Pathogenecity of Aeromonas salmonicida was studied in healthy fish. The inoculum was injected into healthy fingerlings through intramuscular injection and immersion challenge. Histopathological investigation of the infected organs was also studied. The bacterial infection was treated using neem nanoemulsion by in vitro and in vivo analysis and was found to be effective. © 2013 Elsevier B.V.