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Sequence variability in three mitochondrial DNA (mtDNA) regions, namely portions of cytochrome c oxidase subunit 1 (p cox1), NADH dehydrogenase subunit 1 (p nad1) and NADH dehydrogenase subunit 4 (p nad4), for Toxocara canis. Baylisacaris transfuga. Ascaris suum and Parascaris equorum from Canis lupus. Ursus thibetanus. Sus scrofa and Equus burchelli in China were examined. The lengths of the sequences of p cox1, p nad1 and p nad4 were 711 bp, 648 bp and 666 bp, respectively. No intra-species differences were detected in p cox1 for the four examined ascarid species, in p nad1 for T. canis. A. suum and P. equorum, and in p nad4 for B. transfuga and P. equorum. Sequence differences in p nad4 for six roundworm samples of T. canis and P. equorum were 0-0.1% and 0-0.3%, respectively, and were 0-0.3% in p nad1 for six roundworm samples isolate of B. transfuga. The inter-specific sequence differences among four species were 8.7-12.4% for p cox1, 13.9-17.7% for p nad1, and 14.0-25.7% for p nad4. Phylogenetic analyses suggested that the three mtDNA fragments could be used to identify ascarid species in families Ascaridiae and Toxocaridae. [ABSTRACT FROM AUTHOR]