An enzyme-linked immunosorbent assay (ELISA) based on a recombinant rhoptry-associated protein-1 (RAP-1) of Babesia bovis has been previously developed, but it was imperfect because some cross-reactions were still present in Babesia bigemina-infected bovine sera. To improve its accuracy for the specific detection of the antibodies to B. bovis, we constructed three C-terminal truncated recombinant antigens of the RAP-1-rCT1 (amino acids [aa] 301 to 408), rCT2 (aa 388 to 490), and rCT3 (aa 466 to 565)-by using a baculovirus expression system and evaluated their diagnostic potentials using ELISA. rCT1 and rCT2 were better diagnostic antigens in their sensitivities and diagnostic efficiencies than rCT3, although none of the recombinant antigens showed any cross-reactivity to B. bigemina-infected bovine sera. These results confirmed that the N-terminal 300-aa region caused cross-reactivity of the entire RAP-1 antigen, and the C-terminal truncated recombinant antigens were shown to be useful reagents for species-specific serodiagnosis.