Aims: To develop probiotics for the control of vibriosis caused by Vibrio anguillarum and Vibrio ordalii in finfish. Methods and Results: Kocuria SM1, isolated from the digestive tract of rainbow trout, was administered orally to rainbow trout (Oncorhynchus mykiss) for 2 weeks at a dose equivalent to c. 108 cells per g of feed and then challenged intraperitoneally with V. anguillarum and V. ordalii. Use of SM1 led to a reduction in mortalities to 15-20% compared to 74-80% mortalities in the controls. SM1 stimulated both cellular and humoral immune responses in rainbow trout, by elevation of leucocytes (5 center dot 5 +/- 0 center dot 8 x 106 ml-1 from 3 center dot 7 +/- 0 center dot 8 x 106 ml-1), erythrocytes (1 center dot 2 +/- 0 center dot 1 x 108 ml-1 from 0 center dot 8 +/- 0 center dot 1 x 108 ml-1), protein (23 +/- 4 center dot 4 mg ml-1 from 16 +/- 1 center dot 3 mg ml-1), globulin (15 center dot 7 +/- 0 center dot 2 mg ml-1 from 9 center dot 9 +/- 0 center dot 1 mg ml-1) and albumin (7 center dot 3 +/- 0 center dot 2 mg ml-1 from 6 center dot 1 +/- 0 center dot 1 mg ml-1) levels, upregulation of respiratory burst (0 center dot 05 +/- 0 center dot 01 from 0 center dot 02 +/- 0 center dot 01), complement (56 +/- 7 center dot 2 units ml-1 from 40 +/- 8 center dot 0 units ml-1), lysozyme (920 +/- 128 center dot 8 units ml-1 from 760 +/- 115 center dot 3 units ml-1) and bacterial killing activities. Conclusions: Kocuria SM1 successfully controlled vibriosis in rainbow trout, and the mode of action reflected stimulation of the host innate immune system. Significance and Impact of the Study: Probiotics can contribute a significant role in fish disease control strategies, and their use may replace some of the inhibitory chemicals currently used in fish farms.