In the south-western United States and Mexico, mountain sheep were found to have low levels of mtDNA sequence divergence among haplotypes and low overall nucleotide diversity. Potential causes for this low diversity are discussed. Significant differences in mtDNA haplotype distributions over short distances and high values of NST on a local scale appear to be a result of the tendency of female mountain sheep to disperse less frequently and over shorter distances than males. A lack of concordance between mtDNA haplotype distributions and assumed subspecies boundaries suggest that some taxonomic labels need to be revised.