In 1988, a new plasmid profile was observed for Vibrio salmonicida isolated from cod (Gadus morhua) and Atlantic salmon (Salmo salar) in fish farms in northern Norway. This new plasmid profile, which consisted of plasmids of 61, 21, 3.4, and 2.8 megadaltons, is 1 of 11 plasmid profiles which have so far been observed for V. salmonicida. Plasmid profiling and plasmid DNA hybridization were used in epidemiological studies of cold-water vibriosis. Our results indicate that V. salmonicida was transmitted from Atlantic salmon to cod and vice versa. The 61-megadalton plasmid was found exclusively in V. salmonicida strains originating from northern Norway, which is the only area in which this plasmid has ever been observed. Plasmid DNA hybridization and restriction endonuclease analysis show that the plasmid DNA of V. salmonicida remained stable throughout a 7-year survey.