Piroplasms were found in the blood of Neotoma lepida, Peromyscus californicus, Microtus californicus, Spermophilus beecheyi and Sylvilagus audubonii during wildlife disease surveys in Southern California. Although naturally infected animals appeared healthy, splenectomy of naturally or experimentally infected animals invariably resulted in severe parasitemias and anemias, often terminating fatally. Limited attempts to infect different mammal species were made with varying success. All of the piroplasms were indistinguishable morphologically and were considered to belong to the same species: Babesia microti (Franca, 1912).