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The authors isolated Aeromonas hydrophilla (Biovar I, II, III, IV, and V) from a broad stock of Oreochromis niloticus with 65 % mortality in the winter of 2008, 2009 following their wintering season . There were hemorrhagic patches on the skin, fins, in the mouth and internal organs of affected fish. The bacterial characteristics of the isolated bacteria were studied; experimental infection succeeded only when the bacteria was applied by injection but not via water. Following experimental intraperitoneal infection with the isolate Aeromonas hydrophilla strains No. 5. haemorrhagic patches appeared allover the body of the planktonophagous fishes . The microorganisms were highly virulent to all tested Oreochromis niloticus, as measured in terms of LD50 values. The results of Histopathology revealed the presence of dermal and subdermal hemorrhages, congestion of haemopoeitic organs and the presence of high numbers of immature erythrocytes in the blood vessels. The important role of stress factors in the pathogenesis of the disease was discussed. The economic analysis revealed that the Aeromonas hydrophila causes a great economic losses to fish production farms as its causes a high mortality among the infected fish that reached to 65 % , with decreasing the weight of the fish sale that may reached to 162.5 Kg/100 fish with decreasing the economic returns by 1625 LE/100 fish.