Yersinia pestis, the plague bacillus, has a plasminogen activator (pla) gene on the 9.5-kb plasmid pPla that is hypothesized to play a role in producing the foregut blockage in the flea vector that precedes transmission. In this study, however, Y. pestis that lacked pPla, the 70-kb virulence plasmid, or both plasmids, proved able to block Xenopsylla cheopis fleas normally. Blockage rates decreased with increasing environmental temperature for fleas infected with either wild type or pPla- Y. pestis. Thus, procoagulant ability of the Y. pestis pla gene product does not mediate blockage, nor does its ability to induce fibrinolysis at>28 degreesC account for failure to block at elevated temperatures. A Y. pestis strain that lacked all or part of the third plasmid of 110 kb, however, failed to colonize the flea midgut normally, indicating that one or more genes on the large plasmid may be required for vectorborne transmission.