Chimeric primers, the sensitivity and specificity of which allow them to be used in both the clinical setting and the epizootological assessment of tick infection by a real-time polymerase chain reaction (RT-PCR) assay, have been designed against Babesia canis infection. The findings suggest that a large number of Babesia DNA copies are detectable in the blood in acute babesiosis. Some animals that had experienced babesiosis developed blood B. canis carriage--a small number oftrophozoites remained alive for a long time. When babesiosis was suspected, its diagnosis could be confirmed by RT-PCR in half of dogs with subclinical signs. The tick concentration of Babesia ranged from several hundred to a few thousand parasites. There were no significant differences in the number of Babesia parasites in the infected ticks in relation to their collection site. However, the occurrence of infected ticks was significantly higher in the places of constant contact with a canine population, which is indicative of the decisive role of dogs in the intensity of an epizootic process in the foci of B. canis infection.