Acriflavine-resistant Erysipelothrix rhusiopathiae isolates, defined as serovar la, from arthritic pigs at slaughter houses in Ishikawa prefecture between 1994 and 1995 were compared pathologically and genetically with acriflavine-fast attenuated strain of E. rhusiopathiae used for live vaccine. Twelve of 18 field isolates were resistant to 0.01 % acriflavine similarly to the vaccine strain. Four of the above 12 isolates showed the pathogenicity for mice similar to the vaccine strain. Only one of 7 isolates representative to the above 12 strains showed the pathogenicity for swine similar to the vaccine strain and an identical pattern with the vaccine strain in pulsed-field gel electrophoresis (PFGE). However, the remaining 6 isolates were more pathogenic for pigs than the vaccine strain and showed 4 patterns different from the vaccine strain in PFGE. The present results indicated the incidence of acriflavine-resistant avirulent E. rhusiopathiae strains in the field and pathological and genetical variety of those strains. An appropriate marker other than acriflavine resistance for the vaccine strain must be established. (author abst.)