Babesia ernestoi, the second specie of this genus described in South American opossums, have your characterization based on 2 morphological (dimension and stage morphology) and 2 biological aspects (parasitemy level and host specificity). These aspects are discussed and compared with those of Babesia brasiliensis, the other Babesiidae parasite of these hosts, being proposed that B. ernestoi be considered junior synonym of Babesia brasiliensis.