. Experimental studies of the pathogencity of A.hydrophila, V.anguillarum and E.tarda constituting the most bacterial pathogens of fish in Tilapia species were done. . A. hydrophila was more pathogenic using intraperitoneal route (60.0%) than intramuscular and oral administration (40.0% each). There was no variation in the clinico-pathological pictures between the three routes of infection. The changes in the skin, muscles, liver spleen, kidneys and intestine were described in details. . Intraperitoneal infection with V. anguillarum was more effective in producing lesions (60.0%) than the intramuscular (50.0%) and oral routes (40.0%). Deaths occurred usually between 3-20 days post-infection. Red spots and ulcers under the scales, congestion of the base of the fins. Liver was anaemic with minute petechial haemorrhages. Kidneys were dark red, large and friable in consistency and intestine was within norma1 limits of most cases. . E. tarda injected intraperitoneally caused mortality rate reached 50.0%, while by intramuscular and oral routes each incidence was 30%. The clinico-pathological signs were described in details. . Regarding the re-isolation of infected micro-organisms from the individual organs from dead and survived fish, the liver, kidneys and muscles were the most luxuriant predilection seats. Also, skin, gills and intestinal contents were the best organs affected through the oral administration. . Re-isolation rates were higher among fish infected intraperitoneally reaching 70.0%, 80.0% and 60.0% with A. hydrophila, V. anguillarum and E. tarda respectively.