: Over the past few years, the syndrome of streptococcosis has been associated with outbreaks in
rainbow trout (Oncorhynchus mykiss) and caused important economic losses in the aquaculture industry in
Iran especially in Mazandaran province. The main purpose of this work was molecular identification of some
causative agents of streptococcosis in rainbow trout with emphasis histopathological and molecular
identification of some causative agent of streptococcusis isolated from farmed rainbow trout (Oncorhynchus
mykiss) in Mazandaran province on the north of Iran. The suspicious samples were also provided from a five
contaminated Rainbow Trout farm in Mazandaran Province. Histopathological finding in kidney represent
Depletation of hemopoitic tissue, acute necrotizing glomerulonephritis and Nectoric tubular epithelia,
mononuclear cell infiltration, hemorrhage and tubular hyaline degenerations. Histopathological finding in
spleen represent hemorrhagic foci and necrotic area, disarrangement cell structure, Spleen parenchyma from
an infected fish showing multifocal hemosiderin deposition. In the current study, the host and pathogen
bands with 725 bp and 340 bp lengths were observed in all the five samples. The aim of this study was to use
molecular approaches to investigate the relative contributions of composition of rainbow trout infected by
Streptococcusis emphasis whit histopathological lesion in kidney and spleen
