Three bacterial diseases (tailrot, furunculosis and red-spot disease) are described. A Pseudomonas was associated with the first two and a Vibrio with the third. Tailrot and furunculosis could be cured by treating the fish with antibiotics, but red-spot disease could not because of its very rapid progression and late on-set of signs. Ectoparasites probably facilitate the entry of disease organisms. © 1979.