Rickettsia are obligately intracellular bacteria that are transmitted by ticks, lice and fleas. Rickettsia are normally classified into either of two groups based upon particular outer membrane proteins. The typhus group rickettsiae (TG) contain an immunodominant cell surface-exposed protein designated as rickettsial outer membrane protein B or (OmpB). The second type of rickettsiae is designated as the spotted fever group (SFG). In addition to containing OmpB, SFG rickettsiae contain OmpA. Although the true function of OmpA or OmpB has yet to be definitively established, it is thought that these molecules participate in the attachment of the bacteria to the host cell. My goal is to obtain a complete structural biology analysis of the aforementioned cell surface layer proteins and to determine their roles in pathogenesis and immunity. The information obtained will be used experimentally in vaccine development, the design of adhesion/invasion blocking molecules, and identification of sites for drug interventions.