Aim: To develop a probiotic with effectiveness against Aeromonas sp., which was pathogenic to rainbow trout. Methods and Results: When Bacillus subtilis AB1, which was obtained from fish intestine, was administered for 14 days to rainbow trout in feed at a concentration of 10(7) cells per gram either as viable, formalized or sonicated cells or as cell-free supernatant, the fish survived challenge with the pathogen. AB1 stimulated immune parameters, specifically stimulating respiratory burst, serum and gut lysozyme, peroxidase, phagocytic killing, total and alpha 1-antiprotease and lymphocyte populations. Conclusions: Bacillus subtilis AB1 was effective as a probiotic at controlling infections by a fish-pathogenic Aeromonas sp. in rainbow trout. Significance and Impact of the Study: Disease control in fish is possible by means of the oral application of live and inactivated cells and their subcellular components with the mode of action reflecting stimulation of the innate immune response.