Abstract A strain of Carnobacterium sp., isolated from the intestine of Atlantic salmon, was evaluated for potential use as a probiotic for salmonids. In vitro studies demonstrated antagonism against Aeromonas hydrophila, A. salmonicida, Flavobacterium psychrophilum, Photobacterium damselae subsp. piscicida, Streptococcus milleri, Vibrio anguillarum and V. ordalii but not towards Debaryomyces hansenii, Janthinobacterium lividum, V. alginolyticus, V. harveyi or Yersinia ruckeri. Feeding salmonids with diets containing the probiotic revealed that the isolate remained viable in the gastrointestinal tract. After reverting to feeding with control diets, the isolate was re-isolated from the intestine up to 4 and 10 days in fingerlings and fry, respectively. After feeding with the probiotic for 14 days, challenge by cohabitation indicated effectiveness at reducing disease caused by A. salmonicida, V. ordalii and Y. ruckeri but not V. anguillarum.