Abstract: Objective  To determine the substitution saturation of mitochondrial cytochrome c oxidase subunit I（COX1） gene of Angiostrongylus cantonensis.  Methods  One hundred and thirty A. cantonensis adult worms were collected from 33 sampling sites and used to amplify the complete sequence of COX1 gene. The nucleotide diversity，the number of haplotypes and the number of mutations were calculated by DnaSP software after sequence alignment. The distribution of base substitutions was characterized. The level of nucleotide substitution saturation was evaluated by plotting transitions and transversions against pairwise genetic distance.  Results  A total of 130 complete COX1 sequences of 1 577 bp were obtained. One hundred seventy-one nucleotides were found to be variable，resulting in 39 haplotypes with a diversity of 0.8114. The nucleotide diversity was 0.02841. Mutations were evenly distributed along the COX1 gene and did not show significant clustering. The analysis of COX1 gene showed that the amount of substitutions was increasing with the extension of genetic distance，and transitions outnumbered transversions；the increase rate was 0.76，and 0.16，respectively. The substitution rate was markedly different among the three codon positions：the maximum rate was found at the third codon position，followed by the first position，and the second position. The analysis of COX1 gene among 7 members of Metastrongyloidea showed that transversions outnumbered transitions when the genetic distance was more than 0.15. When the number of transitions exceeded 6%，a plateau was reached；while the transvertions increased linearly.  Conclusion  No substitution saturation is found in mitochondrial COX1 gene of A. cantonensis. Therefore， the substitutions at each codon position could be considered in phylogeny analysis.

Key words: Angiostrongylus cantonensis, Mitochondrial gene, COX1, Substitution, Saturation