ABSTRACTTheCampylobacterexcretion patterns of 26 domestic pet dogs were described in a longitudinal study. The dogs entered the study between 3 and 8 months of age and were monitored until 2 years of age. They were tested monthly forCampylobactercarriage in stool samples that were cultured on theCampylobacter-selective media CAT and modified CCDA agar at 37 and 42°C. This study comprised 366 fecal swab samples, of which 278 (76.2%) were found to beCampylobacterpositive, with the following distribution of species: 75.0%Campylobacter upsaliensis, 19.4%Campylobacter jejuni, 2.1%Campylobacter lari, 0.7%Campylobacter coli, and 2.8%Campylobacterspp. Isolates were typed by pulsed-field gel electrophoresis (PFGE) to elucidate the strain excretion pattern. All study dogs excretedCampylobacterspp. during the study period. At 3 months of age, 60% of the dogs carriedCampylobacter, increasing to nearly 100% carriers at 1 year of age, whereafter the carriage rate decreased to 67% at 24 months of age. The PFGE types showed that individual dogs were often colonized by unique strains ofC. upsaliensisfor several months, up to 21 months or longer. TheseC. upsaliensisstrains were either clonal (or underwent concurrent minor mutative changes) or independent strains. In contrast, the excretedC. jejuniisolates were much more diverse and, in most cases, only seen in one sample from each dog. A high degree of diversity among different dogs was seen. We conclude that young domestic pet dogs excretedCampylobacterspp. during the majority of their puppyhood and adolescent period. In generalC. upsaliensisstrains were excreted for months, with short-term interruptions by or cocolonization with other transitoryCampylobacterspp., predominantlyC. jejuni. C. jejuniwas more prevalent in dogs between 3 months and 1 year of age than in dogs between 1 and 2 years of age.