Ten motile Aeromonads, including one Aeromonas hydrophila ATCC 7512, were used. Of the strains, five were isolated from gulls (one A. hydrophila strain and four A. caviae strains) and four from rainbow trout (three A. hydrophila strains and one A. caviae strain). Whole cell protein profiles of these strains were analyzed by SIDS-PAGE. Forty-five pairs were formed in comparing the protein profiles of each strain. Of the pairs, 17 (37.8%) were different, 18 (40%) were unclear and 10 (22.2%) were identical. When the strains belonging to the same species were compared within themselves, four A. caviae strains were found to be identical, while all four A. hydrophila strains were different or of uncertain relationship to the group. One A. hydrophila strain isolated from gulls was found to be identical to four A. caviae strains again isolated from gulls.