Mongooses are indigenous to Africa and Asia and have been introduced elsewhere. In Cuba, Puerto Rico, Grenada, and South Africa they are the major reservoir and vector of rabies. Elsewhere, sporadic cases of mongoose rabies are reported, but dog rabies can mask their importance. Population density probably determines the importance of the mongoose as a reservoir of rabies. In Grenada during a 4-year period, nearly 30% (and in some areas greater than 50%) of mongooses were found to have rabies serum neutralizing antibodies. The annual proportions of mongooses that were serum antibody-positive and virus-positive, respectively, were inversely related. Natural immunity in mongooses probably lasts for life. In contrast with foxes, in mongooses immunity is an important factor in the epizootiology of rabies. Inoculation of mongooses with Evelyn-Rokitnicki-Abelseth (ERA) vaccine induced a good immunologic response and augurs well for the success of vaccination in the field. The effects of wildlife vaccination on animal populations with high levels of natural immunity need to be investigated before costly campaigns are started.