The goals of this research were to better understand the course of infection, pathology, and biology of Anaplasma organisms harbored by white-tailed deer (WTD; Odocoileus virginianus). The study comprised two objectives: 1) to evaluate the ability of WTD to serve as hosts of and as sentinels for A. phagocytophilum, and 2) to characterize an undescribed Anaplasma sp. of WTD. Four WTD were inoculated with a human isolate of A. phagocytophilum and two served as negative controls. All 4 deer developed antibody titers 64, three circulated organisms in blood as detected by polymerase chain reaction (PCR) for over two weeks, one had PCR-positive bone marrow at 66 days, but none appeared clinically ill. These data confirm that WTD are susceptible to infection with a human isolate of A. phagocytophilum and suggest that WTD may maintain circulating organisms for a sufficient time to infect ticks. Their susceptibility and immunologic response render WTD suitable candidates as natural sentinels. To accomplish the second goal, six WTD were inoculated with either a culture isolate of the Anaplasma sp. or with infected deer blood. All six became persistently infected, as determined by PCR of blood, but none appeared clinically ill. Light microscopy revealed tiny, dark, spherical structures in platelets of infected deer and electron microscopy demonstrated membrane-bound, Anaplasma-like organisms. Immunohistochemistry and in situ hybridization localized to Anaplasma sp. organisms in platelets and Anaplasma sp. DNA was amplified from purified platelets. Five deer developed antibodies reactive to Anaplasma sp. antigen and remained seroreactive for ten or more weeks. Slight antigenic cross-reactivity occurred among this Anaplasma sp., A. phagocytophilum, and A. marginale. Phylogenetic analysis demonstrated that a partial gltA gene sequence amplified from this Anaplasma sp. was closest to A. platys gltA sequence, which is concordant with previous analyses of the Anaplasma sp. 16S and GroESL genes. Two attempts to transmit Anaplasma sp. to deer via Amblyomma americanum, a suspected tick vector, were unsuccessful. The ultrastructural, antigenic, and phyogenetic characteristics of this organism confirm that it is a distinct species of Anaplasma which persistently infects WTD platelets. The name Anaplasma odocoilei is proposed for this new species.