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. 10 8 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 (5AE5 ± 0AE8 · 10 6 ml )1 from 3AE7 ± 0AE8 · 10 6 ml )1 ), erythrocytes (1AE2 ± 0AE1 · 10 8 ml )1 from 0AE8 ± 0AE1 · 10 8 ml )1 ), protein (23 ± 4AE4 mg ml )1 from 16 ± 1AE3 mg ml )1 ), globulin (15AE7 ± 0AE2 mg ml )1 from 9AE9 ± 0AE1 mg ml )1 ) and albumin (7AE3 ± 0AE2 mg ml )1 from 6AE1 ± 0AE1 mg ml )1 ) levels, upregulation of respiratory burst (0AE05 ± 0AE01 from 0AE02 ± 0AE01), complement (56 ± 7AE2 units ml )1 from 40 ± 8AE0 units ml )1 ), lysozyme (920 ± 128AE8 units ml )1 from 760 ± 115AE3 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.