To evaluate Enterobacter cloacae and Bacillus mojavensis, isolated from rainbow trout gut in the present study, as a probiotic to control yersiniosis disease. A strain of Ent. cloacae and B. mojavensis, isolated from the digestive tract of rainbow trout had an antagonistic effect to Yersinia ruckeri, which causes yersiniosis. After feeding fish with 1 x 10(8) cells g(-1) probiotic containing feed for 60 days, the fish survival rate increased to 99.2% following challenge with Y. ruckeri compared with controls that had 35% survival rate. Effects of Ent. cloacae and B. mojavensis on weight gains and stimulation of red blood cells, white blood cells, platelets and hemoglobin were also evaluated in separate groups of fish fed probiotic containing feed for 2 months. Probiotic significantly affected white blood cells, hemoglobin and weight gains of the experimental fish. Enterobacter cloacae and B. mojavensis, can be used to prevent and control yersiniosis disease. In conclusion, concomitant use of Ent. cloacae and B. mojavensis as a feed supplement is beneficial to rainbow trout. Use of these organisms can protect fish from yersiniosis and enhance digestibility and utilization of feed. Use of such probiotics may also limit the use of antibiotics and other chemicals in control and treatment of diseases, and thus contribute to the effort to reduce environmental contamination by residual antibiotics and chemicals.