SummaryWe have identified and sequenced anhdcgene in theVibrio anguillarumplasmid pJMl which encodes a histidine decarboxlase enzyme and is an essential component for the biosynthesis of anguibactin. The open reading frame corresponds to a protein of 386 amino acids with a calculated molecular mass of 44 259.69 Da. The amino acid sequence has extensive homology with the pyridoxal‐P‐dependent histidine decarboxylases ofMorganella morganii, Klebsiella planticola, andEnterobacter aerogenes.Tn3‐HoHo1 transposition mutagenesis of thehdcgene present in a recombinant clone carrying the entire pJMI Iron uptake region produced two derivatives, one with thelacZgene in the same orientation as the direction ofhdctranscription and the other with thelacZgene in the opposite orientation. A V.anguillarumstrain harbouring one of the mutated derivatives was unable to grow under iron‐limiting conditions and did not produce anguibactin. Therefore, thehdcgene must play a role in the biosynthetic pathway of this siderophore and consequently in conferring the high virulence phenotype to this bacterium. The role of histidine decarboxylase in biosynthesis of anguibactin was confirmed by the fact that growth under iron starvation was restored by addition of histamine to the medium. The presence of anguibactin was also demonstrated in supernatants from cultures of thehdcmutant strains grown under iron starvation with the addition of histamine, further confirming that histamine is a precursor in the biosynthesis of the siderophore. immunoblot analysis of production of β‐galactosidase byV. anguillarumstrains carrying the lacZ fusions demonstrated that expression of histidine decarboxylase is not regulated by the iron concentration of