Restriction endonuclease fingerprinting (REF) analysis was used to examine total cellular DNA prepared from 56 independent field isolates of the fish pathogen,Aeromonas salmonicida. DNA was digested singly with the restriction enzymesEcoRI andHindIII, and the resulting fragments separated by polyacrylamide gel electrophoresis and visualized by silver staining. The REF patterns of typical isolates ofA.salmonicidasubsp.salmonicidawere distinct from those ofA.hydrophila,A.salmonicidasubsp.achromogenes,A.salmonicidasubsp.masoucida, and atypical isolates ofA.salmonicidasubsp.salmonicida. Differences between strains of typicalA.salmonicidasubsp.salmonicidacould also be distinguished. Canadian isolates examined could be assigned to 1 of 12 different groups (REF groups), with the majority of the isolates belonging to REF groups 1 and 5. REF group 1 strains were isolated from British Columbia and New Brunswick while REF group 5 isolates were found in Ontario. None of the European strains examined had REF patterns identical to those of Canadian isolates. Based on REF analysis, there was little genetic heterogeneity detected among 23 isolates from two short-term studies of naturally occurring infections. Several different REF groups were seen amongA.salmonicidacollected over a 10-year period from coho salmon from the Credit River. Consistent with earlier biochemical and hybridization studies, the REF data suggest thatA.salmonicidais a clonal pathogen. REF analysis can, however, permit the identification of subgroups, which may be useful in epidemiological studies.Key words:Aeromonas salmonicida, restriction fragment length polymorphism, restriction endonuclease fingerprinting, furunculosis, epidemiologic method.