The two rodent intra-arterial nematodes, Angiostrongylus cantonensis and Angiostrongylus costaricensis, can cause human ill-health. The present study aimed to characterize and compare the mitochondrial (mt) genomes of these two species, and clarify their phylogenetic relationship and the position in the phylum Nematoda. The complete mt genomes of A. cantonensis and A. costaricensis are 13,497 and 13,585bp in length, respectively. Hence, they are the smallest in the class of Chromadorea characterized thus far. Like many nematode species in the class of Chromadorea, they encode 12 proteins, 22 transfer RNAs, and two ribosomal RNAs. All genes are located on the same strand. Nucleotide identity of the two mt genomes is 81.6%, ranging from 77.7% to 87.1% in individual gene pairs. Our mt genome-wide analysis identified three major gene arrangement patterns (II-1, II-2, and II-3) from 48 nematode mt genomes. Both patterns II-1 and II-2 are distinct from pattern II-3, which covers the Spirurida, supporting a closer relationship between Ascaridida and Strongylida rather than Spirurida. Thymine (T) was highly concentrated on coding strands in Chromadorea, but balanced between the two strands in Enoplea, probably due to the gene arrangement pattern. Interestingly, the gene arrangement pattern of mt genomes and phylogenetic analysis based on concatenated amino acids indicated a closer relationship between the order Ascaridida and Rhabditida rather than Spirurida as indicated in previous studies. These discrepancies call for new research, reassessing the position of the order of Ascaridida in the phylogenetic tree. Once consolidated, the findings are important for population genetic studies and target identificatio
