Technical Abstract: Recent serologic, immunoprotection, and pathogenesis studies implicate the Lig proteins as key virulence determinants in interactions of leptospiral pathogens with the mammalian host. We examined the sequence variation and recombination patterns of ligA, ligB, and ligC among 10 pathogenic strains. Mean DNA and amino acid sequence variation was 8.4% and 11.1% for ligA, 19.4% and 21.8% for ligB, and 9.1% and 6.3% for ligC, respectively. All strains were found to have intact ligB genes and genetic drift accounted for most of the ligB genetic diversity. Like ligB, ligC gene variation also followed phylogenetic patterns, suggesting an early gene duplication event. However, several strains have inactivated versions of the ligC gene, suggesting that LigC is not essential for virulence. The ligA gene was found exclusively in L. interrogans and L. kirschneri strains, and appears to have been created from ligB by a two-step partial gene duplication process. Two ligB genes and one ligC gene had mosaic compositions. Evidence for recombination events between related bacterial species was also found for some ligA genes. In conclusion, the results presented here indicate that several types of evolutionary mechanisms have been acting on the lig genes, including genetic drift, gene duplication, and horizontal gene transfer.