The large Babesia species of dogs, Babesia canis, is transmitted by different ticks. Dermacentor reticulatus, Rhipicephalus sanguineus and haemaphysalis leachi are the known main vectors. Four B. canis isolates of different geographic origin were investigated for their transmission specificity and pathogenicity in infection trials. R. sanguineus ticks exclusively transmitted the Babesia isolate from Egypt. D. reticulatus was the vector for isolates from Hungary and France. Transmission of an South-African isolate was only possible by H. laechi. The B. canis isolates differed markedly in their pathogenicity. The South-African isolate was highly pathogenic and resulted in nearly complete mortality. Infections with the isolate from Hungary transmitted by D. reticulatus, also resulted in severe clinical disease which often ended fatal without treatment with an anti-Babesia drug. Clinical disease also resulted from infections with the French isolate while the isolate from Egypt was largely non-pathogenic. Challenge trials for investigations of cross-immunity demonstrated immunogenic differences between the individual isolates. The degree of immunogenicity appeared to be related to the pathogenicity of the Babesia isolates. Serological investigations revealed that antigen of the isolate from Hungary in ELISA reacted markedly higher than the other three antigens. Due to their vector specificity B. canis classification into three groups is possible: B. canis canis transmitted by D. reticulatus, B. canis vogeli transmitted by R. sanguineus and B. canis rossi transmitted by H. laechi.