A history of sever mortalities and morbidities among cultured Oreochromis niloticus in many different  freshwater fish farms in Alexandria province were reported.                                                                                                                                     Clinical observation , postmortem changes as well as bacteriological isolation and identification revealed the presence of Aeromonas hydrophila in 60 % of the examined fish samples .      Both intramuscular and intraperitoneal  inoculation of Aeromonas hydrophila in 20 apparently healthy Oreochromis niloticus fish induced the previously observed clinical signs and re-isolation of the injected pathogen were done . isolated Aeromonas hydrophila were found to be highly sensitive to Oxytetracyclin and sulphamerazine .