Abstract: Streptococcus agalactiae was isolated and identified as the pathogen from diseased Nile tilapia(Oreochromis niloticus)populations in China.The pathogen was gram stained through a standard protocol,negative stained and then photoed under electron microscopy.Tissue samples from infected fish were collected and examined for pathological changes under optical and transmission electron microscopes to elucidate the mechanism of infection of Streptococcus agalactiae in tilapia.The result showed that the pathogen was gram positive.And sphere-shaped bacteria were alone or arranged in chains.The most marked pathology observed was severe congestion in the liver,spleen and kidney.Edema,degeneration and acute multifocal necrosis were commonly observed in many organs.There was also evidence of an infiltration of numerous inflammatory cells.Ultrastructural pathology examination indicated that S.agalactiae were widely distributed in many internal organs,especially in the spleen.Moreover,a series of pathological changes were found in the target tissue cells.The boundaries between the cellular compartments were less defined,resulting in the loss of a normal cytoplasmic compartmentalization.There were numerous cases in which cells containing aberrant nuclei appeared singly or in clusters.The cytoplasm often contained degenerated mitochondria,which swelled with disintegration and lyses of the cristaes.Degranulation of the rough endoplasmic reticulum as well as dilatation of the reticulum cisternae was also prevalent.Cells containing such features were frequently accompanied by vacuolar degeneration in cytoplasm.Cardiac and dorsal muscle fibers were fractured and disordered.Sarcomere length of dorsal fiber bundles differed.Microvillus arranged disorderly and in different length.Fibrin precipitated in necrotic foci of the eye.These observations indicated that S.agalactiae can cause simultaneous development of multiple-organ lesions with an acute systemic inflammation in the host,especially in the liver,spleen,kidney and brain,which leads to the death of the fish.