Aims: The aim of this study was to assess the efficacy of in-feed probiotics as a preventive measure against skin infections caused by Aeromonas bestiarum and Ichthyophthirius multifiliis (Ich) in rainbow trout. Methods and Results: Fin rot was induced in fish by intradermal injection with 0.1 ml volumes containing 10(5) cells per ml A. bestiarum at the base of the dorsal fin. Ich infections resulted from immersion in Ich-contaminated water. Each probiotic was administered orally [10(8) cells per g feed for GC2 (Aeromonas sobria) and 10(10) cells per g feed for BA211 (Brochothrix thermosphacta)] for 14 days. Results showed that, after challenge with A. bestiarum, probiotics GC2 and BA211 led to 76% and 88% survival, respectively, in contrast to 22% survival for controls. Fish fed with probiotic GC2 had 100% survival after challenge with Ich compared with 2% for probiotic BA211 and 0% for controls. Analysis of innate immune responses revealed that probiotic GC2 promoted higher phagocytic activity, whereas probiotic BA211 led to enhanced respiratory burst activity. Conclusion: Of the two probiotics examined, GC2 was more effective in protecting against both fin rot and Ich. Each probiotic appeared to stimulate different pathways within the innate immune system. Significance and Impact of the Study: This is the first demonstration that probiotics can protect fish against surface infections. Furthermore, this is the first time a probiotic has been shown to protect against a eucaryotic pathogen, namely I. multifiliis.