Americans in South Vietnam during the period 1966–1971 were exposed to rats and fleas (Xenopsylla cheopis) that were infected with both Yersinia pestis and Rickettsia mooseri. Infections with R. mooseri accounted for most nonmalarial, arthropod-borne infections diagnosed in Americans stationed in cantonment areas, while the incidence of plague in Americans was negligible, even though plague epidemics were occurring in the civilian populations surrounding the cantonment areas. Although a rate of one clinical case of plague per 1,000,000 man-years of exposure was experienced in Vietnam, serologic patterns in. American patients infected with R. mooseri suggested that some subclinical infections with Y. pestis also had occurred. A plausible explanation for the absence of clinical plague in Americans was that the killed plague vaccine (USP) routinely administered to American personnel was highly effective for the prevention of clinical plague.