Edwardsiellosis affected cultured O. niloticus showed haemorrhages allover the body especially at base of pectoral fins, operculum and belly, hemorrhagic mouth, protruded and congested vent, fin rot and exophthalmia. The isolated strains were identified biochemically as citrate negative and citrate positive E. tarda and the identification was confirmed by API20E system and PCR. The citrate negative isolate was highly pathogenic than citrate positive one. The LD50 of citrate negative E. tarda for O. niloticus was 1.5 × 104 CFUml-1 . Probiotic, Enterococcus faecium was used as a food additive as a trial for prevention of edwardsiellosis and its role was not only by colonization and competitive exclusion but also by enhancing the immune response through significant increase in lymphocytes, total protein, albumin, globulin and lysozyme.