Bartonellosis is a disease affecting a variety of animals. Many Bartonella infections are zoonotic, including cat scratch disease. Within the genus Bartonella are 45 species, of which more than 10 can infect cats and dogs. Companion animals serve as reservoirs for several zoonotic species of Bartonella , and may also serve as sentinels for zoonotic Bartonella species harbored by wildlife. The aim of this study was to determine the frequency of the occurrence of Bartonella spp. DNA in dogs from households where cats with clinical bartonellosis were kept. The presence of DNA with 99–100% compliance of the nucleotide sequence with the sequence of the Bartonella DNA isolated from cats was demonstrated in the body of 10% of tested dogs. The results indicate that cats serve as a Bartonella reservoir for dogs, and the dogs can play the same role with regard to humans.