A composition comprises bacteriophage covalently attached to an edible particle and is for use in treating bacterial infection in fish or crustaceans. Infections in fish or crustaceans caused by Vibrio, Aeromonas, Yersinia, Moritella, Rickettsia, Piscirickettsia, Lactococcus, Pseudomonas, Flavobacterium or Photobacterium bacteria species can be treated. Bacteria infected with a lysogenic bacteriophage are used for treating disease of fish or crustaceans caused by similar infections by bacteria carrying lysogenic bacteriophage that express a toxin gene.