An unknown mortality was occurred in some ponds of a farm in an area of Kohgiloyeh-&-Boyerahmad province. Three ponds showed the highest mortality rate, about 5 to 20 dead fish were reported daily for 45 days. Afflicted fish have darkening in the skin and swimming near surface or at the floor of pond and swam unmatched with flock. Some fish showed hemorrhages around and within the oral cavity and exophthalmia occasionally. About 45 moribund rainbow trout were sampled for dissection and bacteriological examination. Kidney, liver and spleen of the fishes were examined for the detection and identification of bacteria causative agent of disease in rainbow trout. They cultivated aseptically on different media (Blood Agar, Tryptic soy Agar, TCBS Agar and MacConkey Agar). All fish have been infected and suffered septicemia. A type of gram's negative bacteria was responsible of infection and mix infection was not confirmed. The bacterium was being able to grow on Mac Conkey agar. It was a bacterium very similar with Yersinia ruckeri which induced infection. Detection was based on clinical finding and bacteriological laboratory results so using of some confirmation methods such as FAT or PCR would be recommended strongly.