Ticks of the genus Ornithodors feed on many rodent species with known roles in maintaining and amplifying plague (Yersinia pestis) in nature. Five species of ticks in this genus are shown to acquire the plague bacillus by feeding on infected hosts. Most notably, 80% of O. hermsi Wheeler, Herms and Meyer harbored Y. pestis 1 mo after an infectious blood meal. Subsequent transmission by infected ticks was not demonstrated. However, 1 yr after the infectious feeding, Y. pestis DNA was detected using a hybridization probe on tick triturates that were amplified by in vitro cultivation. Y. pestis can remain reproductively viable for at least 1 yr in about 2% of ticks fed on infected hosts.