One hundred and twenty-nine Vibrio anguillarum serogroup O2 strains were compared by slide agglutination and Western blotting for their lipopolysaccharide (LPS) structure, The strains showed six different LPS profiles, four different reaction patterns in Western blotting, and four different kinds of reaction in slide agglutination, when both unabsorbed and absorbed anti-O2a and anti-O2b sera were used. All in all, nine different groups were detected when the combination of these three methods was applied. The two serological methods gave corresponding results for almost all strains (96%). Most of these strains (84%) belonged to sere-subgroup O2a, while 12% of the strains belonged to sere-subgroup O2b. The remaining six strains had varying reactions in the used serological methods; therefore, their sero-subgroups could not be determined. These results suggest the existence of additional sere-subgroups within serogroup O2.