Antiserum against lateral flagella (L flagella) of Vibrio fluvialis AQ 0007 was prepared, and H agglutination tests of V. fluvialis strains were carried out with the antiserum. Thirty-seven strains isolated from human patients, marine products, or seawater were tested. All strains, except three which had lost the ability to form L flagella, agglutinated with that antiserum. The antiserum did not agglutinate other vibrio species having L flagella, whereas antisera against L flagella of other vibrios did not agglutinate V. fluvialis. These results suggest that the L flagellar antigen of V. fluvialis is common and specific to the species, and is a useful tool for serological identification of the organism.