Aeromonas salmonicida subsp. salmonicida causes outbreaks of furunculosis in salmonid fish. Furunculosis was first detected in Finland in 1986 on fish farms located on the Finnish coast of the Bothnian Bay. Molecular methods, SDS-PAGE, ribotyping, randomly amplified polymorphic DNA (RAPD) and plasmid profile analysis as well as phenotypic characteristics (biochemical characteristics, maximum growth temperature, pigment and elastase production) were used both for typing the strains and to study the possible routes of transmission of the organism to Finland and the spread of infection within Finland from 1986 to 1993. Ribopattern analysis of chromosomal DNA digested with SmaI, BglI, PstI and ClaI of 28 Finnish strains and eight foreign strains (Denmark, Sweden, Norway or Canada) showed that all Finnish strains and the Swedish strain originating from the Swedish coast of the Bothnian Bay had identical ribopatterns. All other foreign strains had distinct, unique ribotypes except for the second Swedish strain studied, the ribotype of which was identical with that of one Danish strain. RAPD typing, based on the results of two arbitrary primers, yielded 15 types for the Finnish strains. Except for both Danish strains, which had the RAPD type which was identical with that of one Finnish strain, the foreign strains had RAPD patterns differing from those of the Finnish strains. Plasmid profile typing and RAPD profile typing did not correlate. Ribotyping with four different enzymes proved to be the most sensitive method for studying genetically homogeneous Aer. salmonicida subsp. salmonicida. Ribopattern analysis showed that the infection which first started in 1984/1985 on the Swedish coast of the Bothnian Bay may have been transmitted to Finland where the first outbreaks occurred in 1986. The strains infecting Finnish fish farms were very homogeneous, with most differences seen, for example, in maximum growth temperature, plasmid profiles and the RAPD profiles of the strains.