The complete mitochondrial DNA (mtDNA) molecule of the domestic sheep, Ovis aries, was sequenced, together with part of the mtDNA of a specimen representing the other major O. aries haplotype group. The length of the complete ovine mtDNA presented is 16,616 nucleotides (nt). This length is not absolute, however, due to heteroplasmy caused by the occurrence of different numbers of a 75-nt-long tandem repeat in the control region. The sequence data were included in analyses of intraspecific ovine molecular differences, molecular comparisons with bovine mtDNAs, and phylogenetic analyses based on complete mtDNAs. The comparisons with bovine mtDNAs were based on the central domains of the ovine control regions, representing both major ovine haplotype groups, and the corresponding domains of Bos taurus and B. indicus. The comparisons showed that the difference between the bovids was 1.4 times greater than the intraspecific ovine difference. These findings suggest that the strains of wild sheep from which domestic sheep originated were more closely related than were the B. primigenius subspecies which gave rise to B. indicus and B. taurus cattle. Datings based on complete mtDNAs suggest that the bovine and ovine lineages diverged about 30 million years before present. This dating is considerably earlier than that proposed previously.