We tested 276 sera from 18 free-ranging black-tailed and mule deer (Odocoileus hemionus) herds in California (USA) collected from 1987 to 1991 in five biogeographical habitat types, for antibodies against eight infectious disease agents. Overall antibody prevalence was 56% for Anaplasma marginale, 31% for Borrelia burgdorferi, 16% for bluetongue virus serotype 17, 15% for epizootic hemorrhagic disease virus, 7% for Coxiella burnetii and Toxoplasma gondii, respectively, and 0% for bovine leukosis virus and caprine arthritis/encephalitis virus, respectively. Antibodies against Lyme borreliosis and anaplasmosis were found in deer throughout California, but antibodies against bluetongue and epizootic hemorrhagic disease were most prevalent in deer from southern California.