Differential gene expression is a key strategy adopted by the Lyme disease spirochaete,Borrelia burgdorferi,for adaptation and survival in the mammalian host and the tick vector. ManyB. burgdorferisurface lipoproteins fall into two distinct groups according to their expression patterns: one group primarily expressed in the tick and the other group primarily expressed in the mammal. Here, we show that the Fur homologue in this bacterium, also known asBorreliaoxidative stress regulator (BosR), is required for repression of outer surface protein A (OspA) and OspD in the mammal. Furthermore, BosR binds directly to sequences upstream of theospABoperon and theospDgene through recognition of palindromic motifs similar to those recognized by other Fur homologues but with a 1 bp variation in the spacer length. Putative BosR binding sites have been identified upstream of 156B. burgdorferigenes. Some of these genes share the same expression pattern asospAandospD.Most notably, 12 (67%) of the 18 genes previously identified in a genome-wide microarray study to be most significantly repressed in the mammal are among the putative BosR regulon. These data indicate that BosR may directly repress transcription of many genes that are downregulated in the mammal.