Yersinia pestis, the bubonic plague bacillus, is transmitted primarily by the bites of infected fleas. Whereas the molecular pathogenesis of plague in the mammalian host has been the focus of much research, the molecular and genetic mechanisms that enable Y. pestis, uniquely among the Enterobacteriaceae, to utilize a blood-sucking insect for transmission have not been studied. The goal of this project is to identify Y. pestis genes that are specifically expressed in the flea and to understand the molecular mechanisms that underlie each step of infection in the flea, from ingestion in the blood meal to subsequent transmission.