A collection of 124 strains of the bacterial fish pathogen Aeromonas salmonicida subsp. salmoniicda isolated from diseased salmonids in Denmark (n=57), Norway (n=28), Scotland (n=20), and North America (n=19) was characterized by ribotyping. Although Danish and North American strains were genetically very homogenous, Norwegian and Scottish isolates showed some genetic diversity. In general, ribotyping was not found to be applicable as an epizootiological typing system for A. salmonicida subsp. salmonicida due to low discriminatory power.