Borrelia burgdorferi is maintained in an infection cycle between mammalian and arthropod hosts. Appropriate gene expression by B. burgdorferi at different stages of this cycle is probably essential for transmission and establishment of infection. The B. burgdorferi beta(3) integrin ligand P66 is expressed by the bacteria in mammals, laboratory culture, and engorged but not unfed ticks. No in vitro culture conditions in which P66 expression reflected that in the unfed tick were found, suggesting that there are aspects of B. burgdorferi-tick interaction that remain unexplored.