The current study was performed to investigate the role of biological (bacterial) pollutants in disease occurance in Oreochromis niloticus obtained from the western lagoon of Lake Temsah, Ismailia. The naturally infected O.niloticus showed no pathognomonic signs or lesions, except some fish revealed haemorrahage, tail rot with enlarged and different grades of congestion in the internal organs. There were different isolated bacteria as identified Pseudomonas aurginosa, Proteus vulgaris, Enterobacter cloacae,E.coli and salmonella sp. They were identified using ViteK2 system. Ps. aurginosa was confirmed by using PCR. Experimental infection was carried out and reisolation of Ps. aurginosa was tried.