الفهرس 
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Abstract
A total number of two hundreds (200) naturally infected and freshly caught nile Tilapia (oreochromis niloticus) were colleced randomly and seasonally from western lagoon (a branch from lake temsah in Ismaailia governorate). The clinical picture of the affected O.niloticus were recorded then they were examined to isolate and identify the bacteria causing motile aeromonas septicemia. The most isolated bacteria were aeromonas sobria which identified by using traditional methods and vitek system. The identification was confirmed by detection of 16s rRNA gene using PCR. The virulence of aeromonas sobria was determined by detection of hemolytic activity on blood agar and detection of hemolysin gene using PCR. The total prevalence of aeromonas sobria in O.niloticus was 63%, with the highest prevalence in summer (92%) followed by spring (80%) then autumn (60%) while the lowest prevalence was in winter (20%). Aeromonas sobria was isolated with high prevalence from kidneys, liver, spleen and intestine at rate of 25.39%,23.02%, 19.84% and 15.08% respectively. The lowest prevalence was recorded from gills and skin lesions at rate of 10.32% and 6.35%respectively. The experimentally infected O.niloticus by intraperitioneal route (I/P) showed the same signs recorded in the naturally infected fish. The mortality rate represented 80% of the total fish.