Although Leptospira borgpetersenii subtype Hardjobovis and L. interrogans subtype Hardjoprajitno belong to different species, they are serologically indistinguishable and are therefore classified as serovar Hardjo. Since LPS is the major antigen involved in serological classification, this implies that the LPS of these subtypes is identical. Comparison of the LPS biosynthetic loci (rfb) of the subtypes revealed remarkable similarity, with 32 and 31 origins of replication (orfs) in the Hardjoprajitno and Hardjobovis rfb loci, respectively. The order and orientation of these orfs were identical with the exception of an additional orf in Hardjoprajitno between orfs 4 and 5 and intergenic sequences differing between the subtypes. The Hardjoprajitno rfb locus has been divided into four intercalated regions based on sequence similarity to other leptospiral rfb loci. orfJ1-orfJ14 as well as orfJ21-orfJ22 are more similar to regions of the rfb locus of L. borgpetersenii subtype Hardjobovis. orfJ15-orfJ20 as well as orfJ23-orfJ31 are almost identical to the corresponding orfs in L. interrogans serovar Copenhageni. We propose that the progenitor Hardjoprajitno strain, containing an rfb locus which closely resembled the Copenhageni locus, acquired orfs 1-14 and orfs 21-22 from subtype Hardjobovis resulting in two serologically indistinguishable subtypes of serovar Hardjo which in turn constituted the main bovine-adapted leptospiral serovar.