A live, attenuated Yersinia pestis vaccine, designated EV76 (Paris) F, was pathogenic in Cercopithecus aethiops when administered parenterally. Although an oral dose of 10(9) colone-forming units of the vaccine was nonfatal to vervets, a transient but severe gastrointestinal disturbance resulted in four of 12 animals. Seven immunized vervets were protected against intradermal challenge. The remaining five vervets without serologic reactions and five untreated controls died from typical bubonic (septicemic) plague within seven days. General reactions after vaccination were not predictive of later-developing immunity, which appeared to correlate with the production of Fraction I-specific antibodies. Marked differences in immunity were noted with two methods of vaccine administration (incorporation into bananas and dropping into the mouth). This finding supported the prior observation that the oral mucosa is the major portal of entry for this strain of vaccine. Local reactions in the buccal cavity after feeding were negligible. Thus this strain of Y. pestis is a nonlethal, effective weapon against bubonic plague in a susceptible species.