A total of 200 marine fish of two different species represented as (100 Tilapia zillii&100 Mugi
Capito) were freshly collected randomly from different markets in EL-Mansoura city, during different seasons. And subjected to clinical, post mortem and bacteriological studies .The common clinical signs were darkness in skin, hemorrhage in base of fins, eyes &different parts on the body, and abdominal distention, congestion in gills and increase in mucous secretion. The Post Mortem finding showed white serous fluid in abdominal cavity and sometime tinged with blood, pale or congested liver, kidney and spleen. The result indicated that the prevalence of bacterial pathogens among naturally infected marine fish were A. hydrophila
52 isolates (39.39%), V.alginolyticus
38 isolates(28.79%), Ps.fluorescens
24isolate s (18.18%),V. cholerae10 isolates (7.58%) and Ps.aeruginosa8 isolates (6.06%). And among the seasons the highest prevalence of bacterial infection recorded in summer season (33.33%) followed by spring (24.42%), then autumn (21.97%), while minimal prevalence recorded in winter season (20.46%). The results of pathogenicity declared that the A. hydrophila was highly pathogenic to T.zillii causing 100% mortality within 4 days followed by V.alginolyticus causing (90%) mortality, then Ps. Fluorescens(80%), Ps.aeruginosa(60%)while V. cholerae was non pathogenic strain .The results of antibiogram revealed that, ciprofloxacin and naladixic acid were more effective against A .hydrophila, while ciprofloxacin and rifampicine more effective against Ps. fluorescens also ciprofloxacin and amikacine against Ps.aeruginosa .While V.alginolyticus was highly sensitive to ciprofloxacin