Genomic DNA from 15 strains of Helicobacter mustelae was subjected to pulsed-field gel electrophoresis (PFGE) after digestion with PacI and SfiI. H. mustelae genome DNA appeared very similar in all strains examined, whether isolated from ferrets or mink or from animals bred in either the USA or in the UK. The H. mustelae genome size was estimated to be 1.7 Mb, similar in size to that of H. pylori. A minor difference in PacI PFGE pattern and genome size was observed between rifampicin-resistant and rifampicin-susceptible derivatives of H. mustelae F251. Another minor difference in genome pattern based on PFGE with SfiI was observed between an H. mustelae strain used to experimentally infect four ferrets which resulted in loss of an SfiI site in strains obtained from the newly infected ferrets. Thus, although minor differences in PFGE pattern were noted, H. mustelae lacks the genomic diversity observed in H. pylori.