ABSTRACT: The possibility of transmission of Aeromonas salmonicida subsp. salmonicida from Atlantic salmon Salmo salar to cod Gadus morhua, halibut Hippoglossus hippoglossus, and wrasse (Labridae) was studied in both laboratory cohabitation and injection challenge trials, and in a field situation during an outbreak of furunculosis in Atlantic salmon in 2 sea farms containing both cod and salmon. A. salmonicida subsp. salmonicida was isolated from cod, halibut and wrasse, but only occasionally. The injection challenge studies with A. salmonicida subsp. salmonicida were carried out on cod and halibut. Despite challenge with 4.6 x 10³, 4.6 x 10⁴ and 4.6 x 10⁵ CFU (colony-forming units) of the bacterium, only 1 halibut died, whereas no mortality occurred in the cod groups. In cohabitation challenge trials in which cod, halibut and wrasse were kept together with infected salmon, 1 cod and 1 halibut died, with subsequent isolation of A. salmonicida subsp. salmonicida from their kidneys. In a group of salmon challenged in the same way with infected salmon cohabitants, all the salmon died. In the field study, there were high mortalities of salmon due to furunculosis. However, A. salmonicida subsp. salmonicida was isolated from only 1 of the wrasse used as \'cleaner fish\' for the salmon. A. salmonicida was not isolated from cod during the field study. The results suggest that A. salmonicida subsp. salmonicida can be transmitted from salmon and cause disease in cod, halibut and wrasse, but that this is a relatively rare event.