The study was carried out on 1000 O. niloticus collected from private and governmental semi intensive earth ponds in Sharkia province during disease outbreaks, the examined farms belongs to the Central Lab for Aquatic Research in Abbassa as well as private farms through the seasons of the year 2008. The clinical signs and postmortem lesions of diseased fish were recorded. The results revealed that (420) fish were found to be infected with bacterial agents which means that the prevalence of bacterial diseases was 42% among the examined fish. The isolated and identified bacteria were Aeromonas hydrophila in a percentage (56.67%) and the highest rate of infection was observed in winter season (39.9%), Pseudomonas fluorescence in a percentage (36%) and the highest rate of infection was observed in autumn season (48.8%). Streptococcus spp. in a percentage (19.77%) and the highest rate of infection was observed in spring season (54.2%). Flavobacterium columnaris in a percentage (8.34%) and the highest rate of infection was observed in autumn season (57.1%). Pathogencity of selected bacterial isolates belonged to Aeromonas hydrophila or Pseudomonas spp. were done through experimental infection via interperitonial and intramuscular routes. Clinical signs and P.M. lesions were recorded nearly the same clinopathological picture which noticed in naturally infected fish. From this study it could be concluded that: Diseases and resulted mortalities caused by bacterial agents represent important category among diseases affecting O. niloticus under semi intensive culture system in Abbasa Sharkia Province. The most important bacterial diseases affecting O. niloticus under semi intensive culture system in such locality were motile aeromonas septicaemia, pseudomonas septicaemia, streptcoccosis and columnaris respectively. The aqua-culturist and fish farmers must expect the seasonal occurrence of the studied important  bacterial diseases (late winter, autumn and early spring) and all prophylactic measures must be applied beginning from proper water quality, good nutrition, application of effective probiotics and immunostimulants during such periods.