Anaplasma phagocytophilum (formerly Ehrlichia equi) is a tickborne pathogen of domestic horses and the causative agent of equine granulocytic anaplasmosis. After the occurrence of clinical anaplasmosis in a Przewalski's horse (Equus ferus przewalskii) housed at the Smithsonian Conservation Biology Institute in 2008, opportunistic serosurveillance of the herd was initiated. From 2008 to 2014, 57 serum samples were collected from 27 individuals (10 males; 17 females). Using indirect immunofluorescent antibody assays for anti–Anaplasma phagocytophilum antibodies, it was determined that prevalence was 53%. No significant sex differences were identified. A statistical association between increasing age and seropositive status suggests cumulative risk of exposure to Anaplasma phagocytophilum. After exclusion of four clinical cases of anaplasmosis, it was found that 22–57% of those sampled each year were seropositive and clinically normal, suggesting that the majority of Przewalski's horses develop subclinical or self-limiting anaplasmosis after exposure to A. phagocytophilum.