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Gastrointestinal nematodes within the subfamily Ostertagiinae ( Teladorsagia, Ostertagia, and Marshallagia et al.) are among the most common infections of domesticated livestock. These parasites are of particular interest, as many of the species within this group are of economic importance worldwide. Traditionally, nematode species designations have been based on morphological criteria. However, this group possesses poorly defined species. There is an urgent need to develop a reliable technique that can distinguish species of Ostertagiinae. DNA barcoding has been proved to be a powerful tool to identify species of birds, mammals, and arthropods, but this technique has not yet been examined for identifying species of Ostertagiinae. In this study, a total of 138 mitochondrial DNA (mtDNA) cytochrome c oxidase subunit I (COI) sequences from individuals representing 11 species of Ostertagiinae were acquired by PCR for the first time. The specimens were collected from pastoral area of northern China. Genetic divergence analyses showed that mean interspecific Kimura two-parameter distances of COI (13.61 %) were about four times higher than the mean value of the intraspecific divergence (3.69 %). Then, the performance of the COI to identify species of Ostertagiinae was evaluated by identification success rates using nearest neighbor (NN) and BLASTn. The results indicated that the rates of correct sequence identification for COI were high (>80 %) when using the NN and BLASTn methods. Besides, the deep lineage divergences are detected in Teladorsagia circumcincta. Meanwhile, the analyses also detected no genetic differentiation between some species such as Ostertagia hahurica and Ostertagia buriatica. These results indicate that the traditional status of species within Ostertagiinae should be closely examined based on the molecular data. [ABSTRACT FROM AUTHOR]