Rattus rattus, Rattus norvegicus, Rattus exulans, Suncus murinus, and Bandicota sp. trapped alive in Pleiku, Vietnam, were combed for fleas during 1966–68. Only Xenopsylla cheopis were collected from all mammals, and 94% of the fleas were on R. rattus. Montly flea indices showed an inverse correlation with rainfall in Pleiku. Published data on flea indices and reported cases of plague indicated that these were inversely proportional to rainfall in Pleiku, Nha-Trang, and Saigon/Cholon. When the X. cheopis/R. rattus index is 1, 73% of the rats have no fleas. This may be the reason for outbreaks of plague of limited scope, duration, and intensity in the Central Highlands, where flea indices are suppressed by considerable rainfall. During the dry season. the X. cheopis/R. rattus index increases threefold monthly.