The isoenzyme patterns of Paragonimus iloktsuenesis and P. westermani were studied by isoelectric focusing on polyacrylamide gels for PGM, MDH, HK, ME, PO and ACP. Interspecific differences were apparent as shown by the isoenzyme profiles of these enzymes examined. MDH was polymorphic with 2 recognizable phenotypes, designated common type and variant type respectively, in P. i. and with 3 phenotypes, designated MDH1, MDH2 and MDH1-2 respectively, in P. w. The result showed that the two species could be distinguished one from another by such species-specific band patterns of isoenzymes. This suggests that P. w. and P. i. have undergone genetically divergent development and P. i. can be regarded as a valid species different from P. w. In the light of MDH polymorphism, some implications of isoenzyme analysis on taxonomy of the genus Paragonimus were discussed.