We surveyed 1,115 base pairs of mitochondrial DNA sequence from three gene regions in 66 Curve-billed Thrashers (Toxostoma curvirostre) taken from throughout their range. We found that populations sorted unambiguously into three groups. Two of these groups, the Curvirostre and Palmeri groups, have been known for many years because of their distinctive phenotypic characteristics, and their status was recently confirmed with an intensive morphometric analysis. The third (southern) group consists of populations in Puebla and Oaxaca, Mexico. The strong morphological and mtDNA distinctiveness of the Curvirostre and Palmeri groups suggests that they warrant species status. We recommend that more specimens of the southern group be obtained prior to formal taxonomic recognition of this form. Coalescence analyses suggest that the Curvirostre group has undergone a recent population increase, whereas the Palmeri group seems to have been more stable in its Sonoran Desert range.