A monoclonal antibody that had been raised against a protease-containing fraction of Babesia bovis, and shown to bind to a protein located in the rhoptries, was used to screen a B. bovis cDNA expression library. The sequence of the protein encoded by a positive clone was almost identical to the equivalent region of a previously described B. bovis 60-kDa rhoptry protein (Bv60). A tandem repeat of the gene encoding Bv60 was identified in all Australian isolates of B. bovis examined. Genes encoding homologous of Bv60 were cloned from Babesia ovis and Babesia canis. In B. ovis, 5 closely linked genes were identified. Four of these genes appeared to encode very similar proteins (Bo60.1-4). The protein (Bo60.5) encoded by the fifth B. ovis gene had 72% amino acid identity to Bo60.1-4 in the amino-terminal 306 amino acids, but no significant similarities in the carboxy-terminal region. In B. canis one gene (Bc60.2) was sequenced and a second closely linked gene was identified. A further member of the family, p58, has also been described previously from Babesia bigemina. Tandemly repeated genes subject to extensive gene conversion appear to be a feature of this family of babesial rhoptry protein homologous. No proteins significantly related to any members of the gene family were identified in a search of translated DNA and protein sequence databases. Thus the function of this family of proteins remains a matter for speculation.