The hexanucleotide CCAGCA was found repeated 15 times in tandem on the 5' side of the virginiamycin acetyl transferase gene of Yersinia enterocolitica strain Y56. The corresponding region was analysed by PCR from 54 clinical strains belonging to the same biotype and serotype, and others from this laboratory collection belonging to different biotypes and serotypes. Each strain produced a single amplification product whose size was variable among strains, revealing that the locus was polymorphic. Nucleotide sequence determination of selected PCR products showed that the polymorphism was due to the precise expansion or reduction in the number of hexanucleotide repeats. Analysis of this locus in a few strains showing the same PFGE pattern showed that it was also polymorphic. These results suggest that this method could be valuable to increase the discriminatory power of current Y. enterocolitica typing schemes.