Flavobacteruim columnare, causal agent of columnaris disease is pathogenic to many species of freshwater fish throughout the world. The columnaris disease occurred primary on the skin and gills. The Egyptian aquaculture industry is severely impacted by bacterial fish disease. The objective of the present study was to determine the causative agent of an outbreak caused mortality about 15% among cultured Oreochromis niloticus in private fish farm in Behera governorate during late summer of 2008. The clinical signs of infected fish were loss of scales, excessive mucus, white spots on the back and head, skin ulcers, hemorrhage and fin erosion. The isolated bacteria developed characteristic rhizoid yellow pigmented colonies that showing swarming. They was gram–negative, filamentous bacteria exhibiting flexing movements Biochemical characterization proved that the isolated strain could be classified as F.columnare. The in vitro antibiotic sensitivity test showed that oxytetracycline, neomycin, streptomycin, chloramphenicol, kanamycin, lincomycin and erythromycin were drug of choices. Pathogenicity studies were performed on O.niloticus.