Borrelia hermsii, an agent of replapsing fever, avoids the host's immune response by means of multiphasic antigenic variation. Serotype specificity is determined by variable antigens called the Vmp lipoproteins. Through recombination between linear plasmids a formerly silent vmp gene replaces another vmp gene at a telomeric expression locus. We examined strain HS1 borreliae before and after a switch from serotype 7 to serotype 21. The nucleotide sequences of 5 regions of silent and expressed vmp7 and vmp 21 were determined. Silent and active vmp 7 and vmp 21 genes shared a block of homologous sequences surrounding their 5′ ends. Sequences upstream of silent vmp 7 and vmp 21 genes lacked the promoter and substantially differed from each other. In this antigenic switch a vmp gene was activated by a recombination that placed it downstream of a promoter.