ABSTRACTA serotyping scheme based on heat-stable surface antigens was established for 101Campylobacter upsaliensisand 10Campylobacter helveticusstrains isolated from 261 dogs and 46 cats of different ages originating from two geographically distinct regions in Germany. The prevalence ofC. upsaliensisvaried between 27.8% in juvenile dogs (<12 months of age) and 55.4% in adult dogs (P< 0.05). Of the cats, 19.6% harboredC. upsaliensis, whereas 21.7% carriedC. helveticus. Of theC. upsaliensisisolates from both host species, 93.1% belonged to five different serogroups, two of them being prevalent at rates of 47.5 and 27.7%, with different frequencies in both regions. Six (54.6%) of theC. helveticusisolates also belonged to serotypes found amongC. upsaliensisstrains, whereas five (45.4%) possessed an O antigen unique forC. helveticus. In contrast, a considerable degree of genomic diversity of the isolates was assessed by macrorestriction analyses with the endonucleasesSmaI andXhoI, using pulsed-field gel electrophoresis as well as enterobacterial repetitive intergenic consensus sequence PCR (ERIC PCR). Restriction withSmaI pointed towards the existence of clonal groups associated to some extent with serotypes, while restriction withXhoI disintegrated these groups to smaller noncoherent subgroups. Analysis of ERIC PCR profiles did not exhibit any associations with serotypes. In conclusion these data demonstrate the genomic heterogeneity amongC. upsaliensisstrains and indicate that the combination ofSmaI restriction with serotyping is a useful tool to investigate the expansion of clonal groups ofC. upsaliensis.