Technical Abstract: The transition between infection of the mammalian host and colonization of an arthropod vector is required for ongoing transmission of a broad array of pathogens, from viruses to protozoa. Understanding how this transition is mediated provides opportunities to disrupt transmission through either chemotherapy or immunization. We used an unbiased proteomic screen to identify Anaplasma marginale proteins specifically up-regulated in the tick as compared to the mammalian host. Comparative mass spectrometric analysis of proteins separated by two-dimensional gel electrophoresis of uninfected and infected ISE6 cells and infected mammalian cells identified 15 proteins exclusively expressed or up-regulated in tick cells. All 15 had originally been annotated as hypothetical proteins. We confirmed quantitative up-regulation and expression in situ within the midgut epithelial and salivary gland acinar cells of vector ticks during successful transmission. The results support the hypothesis that A. marginale gene expression is regulated by the specific host environment and, in a broader context, that the core genome evolved in the arthropod vector with differential regulation allowing adaptation to mammalian hosts. Furthermore, the confirmation of in situ expression of candidates identified in ISE6 cell lines indicates that this approach may be widely applicable to bacteria in the genera Anaplasma and Ehrlichia, removing a major technical impediment to identification of new targets for vaccine and chemotherapeutic blocking of transmission.