The nucleotide sequences of the mitochondrial DNA (mtDNA) molecules of two nematodes, Caenorhabditis elegans [13,794 nucleotide pairs (ntp)], and Ascaris suum (14,284 ntp) are presented and compared. Each molecule contains the genes for two ribosomal RNAs (s-rRNA and l-rRNA), 22 transfer RNAs (tRNAs) and 12 proteins, all of which are transcribed in the same direction. The protein genes are the same as 12 of the 13 protein genes found in other metazoan mtDNAs: Cyt b, cytochrome b; COI-III, cytochrome c oxidase subunits I-III; ATPase6, Fo ATPase subunit 6; ND1-6 and 4L, NADH dehydrogenase subunits 1-6 and 4L: a gene for ATPase subunit 8, common to other metazoan mtDNAs, has not been identified in nematode mtDNAs. The C. elegans and A. suum mtDNA molecules both include an apparently noncoding sequence that contains runs of AT dinucleotides, and direct and inverted repeats (the AT region: 466 and 886 ntp, respectively). A second, apparently noncoding sequence in the C. elegans and A. suum mtDNA molecules (109 and 117 ntp, respectively) includes a single, hairpin-forming structure. There are only 38 and 89 other intergenic nucleotides in the C. elegans and A. suum mtDNAs, and no introns. Gene arrangements are identical in the C. elegans and A. suum mtDNA molecules except that the AT regions have different relative locations. However, the arrangement of genes in the two nematode mtDNAs differs extensively from gene arrangements in all other sequenced metazoan mtDNAs. Unusual features regarding nematode mitochondrial tRNA genes and mitochondrial protein gene initiation codons, previously described by us, are reviewed. In the C. elegans and A. suum mt-genetic codes, AGA and AGG specify serine, TGA specifies tryptophan and ATA specifies methionine. From considerations of amino acid and nucleotide sequence similarities it appears likely that the C. elegans and A. suum ancestral lines diverged close to the time of divergence of the cow and human ancestral lines, about 80 million years ago.