An experimental Anaplasma marginale infection was induced in a splenectomized mule deer (Odocoileus hemionus hemionus) which persisted subclinically at least 376 days as detected by subinoculation into susceptible cattle. Anaplasma ovis was experimentally transmitted from sheep to a splenectomized and a spleen-intact mule deer, and back to sheep. The pathogenesis in deer was very similar to that seen in sheep using ovine blood inoculations.