Six Lippia sp. were subjected to sequence analysis of the small ribosomal subunit (SRS) or 18S rDNA gene (ca. 1530 bp in length) for studying their genetic relationship. A total of nine nucleotide differences distinguished the species-specific SRS haplotypes of L. alba, L. americana, L. canescens, and L. micromera from L. origanoides and L. graveolens, which shared the same primary SRS haplotype. Five haplotypes (B-F), most with a single nucleotide substitution, were found among 71 naturally collected specimens of L. origanoides. A Neighbor-Joining cluster based on SRS haplotypes from these Lippia sp. and previously characterized related species (Lantana camara, Phyla lanceolata, Verbena bracteata, and V. hastata) indicated two groups, corresponding to Lantaneae and Verbeneae. The Lantaneae group indicated that Phyla lanceolata followed by L. americana were the most distinct species within the clade. In addition, there was evidence of at least three divergent sub-groups within the clade: 1) L. canescens + L. micromera, 2) L. alba + Lantana camara, and 3) L. origanoides + L. graveolens.