In order to ascertain the importance of Campylobacter spp., C.difficile, C.perfringens and Salmonella as agents of bacterial gastroenteritis in dogs, two groups of animals were studied prospectively. The first group consisted of 77 puppies in 14 litters, with fecal cultures performed weekly for 10 weeks, starting at birth. The second group consisted of a kennel population with every dog cultured at entry, and at two-month intervals thereafter. Incidence of Campylobacter spp. was 32 and 31 per 100 dog-month of observation for healthy pups and healthy adult dogs respectively, 46 and 0 for C.difficile, 51 and 36 for C.perfringens and 6.5 and 1.3 for Salmonella. The incidence of Campylobacter spp. in pups peaked at 8 weeks of age. This incidence (43 per 100 dog-months) was higher in pups reared together with older dogs than in pups reared without contact to other dogs (0 per 100 dog-months). Toxigenic strains of C.difficile were found in 61.5% of the healthy neonate dogs. None of the cases of non-watery and non-inflammatory diarrhea we observed was associated with any of the pathogens studied. Furthermore newly acquired colonization with Campylobacter spp. or Salmonella was never associated with episodes of diarrhea. No conclusions could be drawn about the role of bacterial pathogens for causation of watery or inflammatory diarrhea which were not observed in our study.