Abstract: This paper describes the symptoms of an epizootic in yearling rainbow trout caused by E. coli and an epidemic in a fish farm in Shanxi Province. The acute infected disease was characterized by darkening of body colour, reddening of anus and some fins and anemia in gill filaments. Internally, the intestine, liver and spleen were congestive and hemorrhagic. All bacterial isolates were Gram-negative. They were rod-like in shape with peritrichous flagella and fimbriae (pili), measuring 0.9—1.2×2.3—4.5μm. The bacteria were mobile, oxydase negative, M. R. and Indol positive, V. P., and urease and phenylalanine deaminase negative. They could produce acid and gas from glucose and reduce nitrate to nitrite, but did not utilize sodium malonate. Tests on production of hydrogen sulfide, liquafaction of gelatin and growth in KCN medium were negative. Moreover, they fermented D-sorbitol, D-mannitol and L-arabinose, but could not fermentt adonitol, lactose or cellobiose. The Mol% G+C of the DNA is 51.04(Tm). The isolated organisms were identified as Escherichia coli and were differentfac Enterobacteriaceae from any formerly established species of the familyfac that act as causative agents of fish disease.