Ticks serve as vectors of a variety of microorganisms. Among these are the spotted fever group (SFG) rickettsiae, which have developed an association with the vector host to facilitate long-term maintenance by transstadial and transovarial transmission. While surveys have identified a number of SFG rickettsiae in Dermacentor variabilis ticks, the vector competence and effects of rickettsial infection in these ticks remains undefined. Preliminary experiments in our laboratory have provided insights into the ability of D. variabilis to maintain infection with different rickettsial species. We hypothesize that these ticks serve as principle host/vector for long-term maintenance of certain rickettsial infections, while they are refractory to other species. We propose to extend our studies to examine the vectorial competence of D. variabilis for four species (pathogenic and nonpathogenic) of rickettsiae; namely, Rickettsia rickettsii, R. montana, R. peacockii, R. rhipicephali, reported to infect these ticks in nature, and examine the response to infection at the molecular level. Ticks will be experimentally infected with different species of rickettsiae via capillary feeding, and efficiency of transovarial transmission of rickettsiae, as well as tick viability parameters will be assessed. The rate of rickettsial infection in tick progeny over time will be determined by PCR/RFLP-analysis In addition, the response to rickettsial infection in ticks will be addressed by examining differential gene expression. Together, these experiments will further our understanding of the tick-rickettsiae relationship.