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This is the first report on the world that Pseudomonas psychrophila is a new etiological agent of disease in fish. A disease case occurred during the fall of 2017 in a rainbow trout farm in the north of Iran (Guilan province). Anorexia, lethargy and mortalities of 10% fish per week were observed in rainbow trout Oncorhynchus mykiss weighting 45-107g in three fresh water ponds of a farm on the north of Iran. The fish showed external signs of dark pigmentation, exophthalmia, and hemorrhage at the base of the fins and tissues damage. Internal signs were enlarged spleen, pale Liver and intestine filled with yellowish fluid. Kidney and liver of diseased fish were aseptically streaked on MacConkey and sheep blood agar. The cultured bacterial 16S rRNA gene was sequenced. Causative bacteria were identified as pseudomonas psychrophila using morphological and biochemical characteristics and genotypic method based on the 16S rRNA gene.