In this study, clinical findings with macroscopic and microscopic pathologic changes of the tissues and organs after necropsy examination at the 1(st), 2(nd), 3(rd), 5(th) and 7(th) days following intraperitoneally injection of Nile tilapia (Oreochromis niloticus) with Aeromonas hydrophila was aimed to be observed. Clinically; weakness, anorexia, swimming closer to surface, darkness in color with hyperemia and lysis of the fins were observed. In the macroscopic examination the liver was seen to be yellowish brown and crispy with haemorrhagic and greyish white foci on the surface. The gall bladder was tightly full with emerald green bile. The kidney and hearth had haemorrhagic foci whilst intestine lumina observed to be filled with yellow coloured mucoid liquid. The degenerative changes, cytoplasmic fat vacuols and lymphocyte infiltration in lever with focal necrosis of hepatocytes and pancreatic cells were observed histologically. The skin, spleen, gill and eyes did not reveal any significant symptoms. The haemorrhagies and intensive lymphocyte infiltrations in liver, kidney and hearth both in macroscopic and microscopic levels revealed a visceral haemorrhagic septicemia. In this sense; clinical, macroscopic and microscopic findings observed during the present study complied with the Aeromonas hydrophila infections recorded in the other fresh water fishes.