Increased serum iron concentration has been proposed one of the most important predisposing factors to V. vulnificus septicemia. For effective uptake of serum from its environment, V. vulnificus produces siderophores. We tried to compare the growth and siderophore production of several highly virulent and less virulent strains by using the newly developed CAS agar diffusion assay. Highly virulent strains, C7184 and CN35, grew more actively than less virulent strains, A1402 and CN7, in the deferrated Synbase medium. The highly virulent strains produced more siderophores compared with less virulent strains. The culture supernatants obtained from the virulent strains, C7184 and CN35, stimulated the growth of the less virulent strains as well as themselves while the supernatants from the less virulent strains, A1402 and CN7, did not stimulate growth of themselves as well as the highly virulent strains. Growth stimulation was significantly correlated with the total amount of produced siderophores and especially the phenolate siderophore. These results suggested the production of siderophores, especially the phenolate siderophore, plays an important role in the growth of V. vulnificus by uptaking iron under iron-restricted conditions, and the CAS agar diffusion assay proved as a useful method for the quantitative comparison of siderophore production among different pathogens.