Glutathione S-transferase (GST) is a component of a second line of defense against bioreactive radicals derived from host immune attack. Paragonimus westermani causes acute or chronic lung diseases in mammals. A cDNA clone, PwGST#11, of adult P. westermani produced in the present study was 748 bp long and encoded an open reading frame of 217 amino acids with a starting methionine. The molecular mass of this putative polypeptide, Pw26GST, was estimated to be 25.1 kDa with an isoelectric point of 5.7. Pw26GST was homologous with the 26-kDa GSTs of trematodes and vertebrates. Nine of the ten amino acid residues lining the glutathione-binding pocket were conserved. Putative Pw26GST polypeptide was clustered with 26-kDa GSTs of trematodes belonging to the class mu. Recombinant Pw26GST protein generated bacterially, revealed GST enzyme activity toward an universal and class mu-specific substrates. Mouse antisera to recombinant Pw26GST protein recognized native 26-kDa GST of P. westermani but not the GSTs of any other trematodes. Collectively, Pw26GST was found to be a member of class mu GSTs of P. westermani.