Previously, we had identified non-OspA-OspB surface proteins of Borrelia burgdorferi that are targeted by the antibody-dependent complement-mediated killing mechanism. Here we demonstrate by Western blotting that one of these proteins, P35, is upregulated at the onset of stationary phase in vitro. Northern analysis revealed that the upregulation of P35 is at the level of transcription. In addition, the expression of an open reading frame (ORF) located downstream of the p35 gene was found to be regulated in the same fashion as that of P35. This ORF encodes a 7.5-kDa lipoprotein. The transcriptional start sites for both of these genes were determined, to aid in the identification of the putative promoter regions. Additional sequencing of the 5' flanking region of the p35 gene revealed a region of dyad symmetry 52 bp upstream of the transcription start site. Southern analysis demonstrated that the expression of these genes was not due to a cell-density-dependent rearrangement in the genome of B. burgdorferi. These findings provide an in vitro model for studying mechanisms of gene regulation in B. burgdorferi.