Four surface membrane proteins of Babesia rodhaini have previously been shown to induce a degree of protective immunity, and to carry both unique and cross-reactive determinants. cDNA clones for two of the genes coding for these proteins have been isolated and used as probes to isolate a single large genomic DNA fragment which contained all four genes. DNA sequence of two of the genes and their predicted amino acid sequences confirmed that the proteins had hydrophobic sequences at their N- and C-termini, an observation consistent with their proposed cell surface location. Homologies in both amino acid and nucleotide sequences were found at the 3'and at the 5' ends, but considerable sequence variations existed elsewhere in the genes and their products. The genes coding for these four proteins were tandemly arranged along a single relatively short length of chromosome, and such structures, because of their sequence homologies, probably could have arisen by gene duplication. The extensive variation suggested that there may be a functional need for these proteins to be different or capable of varying, although computer analysis implied that the extent of this variation may be constrained by structural requirements. This variation could be indicative of a role for these proteins in the host-parasite relationship or immune evasion.