When Vibrio parahaemolyticus AQ 3627 was grown in the presence of 1,3-diaminopropan-2-ol (OH-Dap), a new compound accumulated in the cells. This was identified as hydroxynorspermidine (OH-Nspd), N-(3-aminopropyl)-1,3-diaminopropan-2-ol, by gas chromatography-mass spectrometry and thin-layer chromatography. It was also synthesized enzymatically from OH-Dap by a cell-free preparation from this strain. All other Vibrio strains examined also showed the ability to synthesize this compound, but none of the non-vibrio organisms did, indicating that OH-Dap is an in vivo substrate for the enzyme responsible for biosynthesis of norspermidine characteristically present in vibrios. These results suggest that the ability to synthesize OH-Nspd from OH-Dap present in the growth medium may be useful as an additional identifying factor for the genus Vibrio.