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A field study on Streptococcus infection as a major cause for epidemic infections in Nile Tilapia aquaculture in Kafr El-Shikh Governorate was conducted. This study was carried out to study the gross lesions and post mortem finding of Streptococcsis in Nile Tilapia. Streptococcus was isolated, then identified by Electron Microscopy (Scanning Electron Microscope (SEM), Energy Dispersive X-Ray Analysis (EDAX) and Transmission Electron Microscope (TEM)) to investigate the phenotypic characteristics, ultra-structure of the bacterial cells. Clinical signs of the infected fish showed presence of hemorrhagic patches on different external body regions, ascitis and exophthalmia. Post mortem examinations revealed presence of inflammation of the liver, spleen and kidney. Microbial identification by Gram stain showed Gram positive cocci in shape arranged in chains and the biochemical tests displayed negative catalase and oxidase tests. Electron Microscopy (SEM, EDAX & TEM) confirmed these results and showed the clear cocci shape of streptococcus species and the arrangement of the chains with a bacterial average length of 700 nm. The EDAX results revealed that the elemental structure of the bacterial sample were C,N,O with the concentration 46.93%, 16.61% and 36.46%, respectively.

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