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Analyses ofmitochondrial (mt) genome sequences in recent years challenge the current working hypothesis of Nematoda phylogeny proposed from morphology,ecology and nuclear small subunit rRNA gene sequences,and raise the need to sequence additional mt genomes for a broad range of nematode lineages.We sequenced the complete mt genomes of three Ascaridia species,namely A.galli,A.columbae and Ascaridia sp.(GHL:2012),that infest chickens,pigeons and parrots,respectively.These three Ascaridia species have an identical arrangement ofmt genes to each other but differ substantially from other nematodes.Phylogenetic analyses of the mt genome sequences of the Ascaridia species,together with 62 other nematode species,support the monophylies of seven high:level taxa of the phylum Nematoda: 1) the subclass Dorylaimia; 2) the orders Rhabditida,Trichinellida and Mermithida; 3) the suborder Rhabditina; and 4) the infraorders Spiruromorpha and Oxyuridomorpha.Analyses ofmt genome sequences,however,reject the monophylies of the suborders Spirurina and Tylenchina,and the infraorders Rhabditomorpha,Panagrolaimomorpha and Tylenchomorpha.Monophyly of the infraorder Ascaridomorpha varies depending on the methods ofphylogenetic analysis.The Ascaridomorpha was more closely related to the infraorders Rhabditomorpha and Diplogasteromorpha (suborder Rhabditina) than they were to the other two infraorders of the Spirurina: Oxyuridorpha and Spiruromorpha.The closer relationship among Ascaridomorpha,Rhabditomorpha and Diplogasteromorpha was also supported by a shared common pattern ofmitochondrial gene arrangement.Analyses of mitochondrial genome sequences and gene arrangement has provided novel insights into the phylogenetic relationships among several maj or lineages of nematodes.Many lineages of nematodes,however,are underrepresented or not represented in these analyses.Expanding taxon sampling is necessary for future phylogenetic studies of nematodes with mt genome sequences.