Enteric redmouth disease is a serious infectious disease in salmonids that causes important economic losses in many countries. Since last few years there were reports on the occurrence of the disease in Iran based on the clinical signs. The disease caused great losses in rainbow trout farms in Fars Province, Iran. Biochemical test of the isolated Yersinia ruckeri resembled high degree of similarity between the isolate and the Yersinia ruckeri cited in the literature as serotype O1. Diseased fish showed typical hemorrhages, blackening of skin, exophthalmia and wide hemorrhages on internal organs. By histopathological examination, diseased fish showed generalized bacterimia with inflammation, glomerulonephritis and necrotic foci in kidney, necrosis and mild telangiectasis in liver, telangiectasis in spleen, hemorrhages and hyperemia in the intestinal mucosa, myocardial degeneration, atrophy and edema in heart, atrophy of pancrease tissues and mucus cell and melanophores hyperplasia in skin of the infected fish.