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Streptococcusis is the one of the most important bacterial fish diseases with outbreak in rainbow trout farms in Iran. The fish farmers have been largely suffered from huge economic losses due to the Streptococcusis outbreaks in different rainbow trout farms in Iran. The present study assessed the effects of some environmental risk factors on incidence of streptococcusis in rainbow trout farms in Haraz River in Mazandaran Province, Iran. A suit of environmental factors including water temperature, nitrite, nitrate, ammonium, water turbidity, DO, water Debi and total count of bacteria were explored as influential factors. Fish and water samples were randomly collected from 10 farm on a monthly basis throughout a year. Isolation and recognition of strep strains were made using biochemical and PCR tests and the data were analyzed by logistic regression method. According to the results, 20% of the differences were explained by the logistic model. Management of these factors might decline the rate of disease outbreak.