A circumscribed natural outbreak of a highly fatal disease of deer, which we have designated epizootic hemorrhagic disease (EHD), has been studied. The disease has proven readily transmissible in deer but not in other experimental or domestic animals tested, nor in embryonating eggs or deer kidney cell cultures. The causative agent is a virus which is readily filterable and is capable of storage, either frozen or in glycerol, for relatively long periods of time. It produces a solid immunity in the few animals that survive and the blood sera of such convalescent animals contain virus-neutralizing antibodies. The disease is one in which large and small hemorrhages occur in both the viscera and skeletal structures of the body, as well as in the subcutaneous tissues. It is probably the same as one known popularly in the southeastern United States as "black tongue" of deer. It is unrelated to epidemic hemorrhagic fever of man or to the disease caused in horses by the equine arteritis virus. At least two serologically different types of EHD virus exist. The New Jersey strain is of greater lethality for experimental deer than the serologically different one obtained from an outbreak that occurred in South Dakota a year after the New Jersey epizootic.