Ascaris lumbricoides (A. lumbricoides) and Ascaris suum (A. suum) were collected from different areas in China to examine the sequence variation of mitochondrial NAPH dehydrogenase subuit 5 gene (nad5) by amplifying partial nad5 (pnad5) followed by sequence analysis. The results showed that the length of pnad5 sequences was 556 bp, sequence differences in the pnad5 was 0.0%2.6% between the A. lumbricoides and A. suum samples. These results support that the A. lumbricoides and A. suum represent the same species.