To clarify the genetic relationships between Clonorchis sinensis and Opisthorchis viverrini, patterns of inter-/intraspecific polymorphism were compared for four markers with nuclear ribosomal DNA (rDNA) and mitochondrial DNA (mtDNA) in liver flukes C. sinensis from Korea (Kimhae) and China (Shenyang and Nanning) and O. viverrini from Laos (Savannakhet). Intra- and interspecific variations in the 18S, ITS2, and 28S rDNA and mitochondrial cytochrome c oxidase subunit I (mtCOI) of mtDNA gene sequences were low and nearly identical. Three isolates of C. sinensis showed a high similarity (99-100%). No variation was detectable in the ITS2 sequence for the C. sinensis from Korea and China. ITS2 region sequences of O. viverrini vs C. sinensis showed 95% identity and differed at 28 nucleotide positions. Pairwise sequence divergence with three C. sinensis isolates and O. viverrini ranged from 0 to 3.94% in mtCOI gene. The mtCOI sequences were more highly conserved relative to the ITS2 sequences. These genetic data from different geographical areas showed that the liver flukes are not variable and are virtually identical almost despite belonging to entirely different genera.