Whether Edwardsiella tarda produces siderophore or not was investigated. CAS assay was used for detecting siderophore and Eagle's MEM was employed as a basal medium with a slight modification. Iron was eliminated from the basal medium by chloroform-8-hydroxyquinoline extraction. On this iron deficient agar medium, all the strains of E. tarda showed siderophore positive reactions. The siderophores were prepared from two virulent and two avirulent strains and examined their electrophoretic mobilities at pH 5.6. The siderophore mobilities against H2PO4-were all-0.05 and no differece was found among these 4 strains.