Blood was collected at slaughter from 132 adult American bison (Bison bison) raised in an anaplasmosis endemic area where the vector Dermacentor andersoni (= venustus) is indigenous. Hematologic studies revealed no indications of clinical anaplasmosis. Card agglutination and complement-fixation tests on all bison serums were negative. Eleven anaplasmosis-susceptible calves each inoculated with 204 ml of blood pooled from 12 bison did not develop anaplasmosis. Results of this study indicate American bison have resistance to natural A. marginale infection.