Polyamine contents of 29 strains of the genus Vibrio, together with 5 strains of other Vibrionaceae, have been determined by a gas chromatographic method to enlarge our knowledge of the occurrence of an "unusual" polyamine, norspermidine, in the genus Vibrio. The results showed that norspermidine was widely distributed in various Vibrio species, except Vibrio fischeri and V. costicola. Besides Vibrio species, Photobacterium leiognathi and P. angustum in Vibrionaceae also contained norspermidine. When the Vibrio strains were grown in the medium supplemented with 1,3-diaminopropane, they synthesized much larger amounts of norspermidine. This ability was specific to the strains which contained norspermidine when grown in the defined medium. These findings suggested that norspermidine, as well as the ability to synthesize much larger amounts of it in the presence of 1,3-diaminopropane, may be useful generic guides to the genus Vibrio.