The number of genomic DNA or cDNA sequences of Schistosoma mekongi accessible in Genbank or EMBL is very limited up to now. Recently, two reports have appeared on the molecular phylogeny of Schistosoma species inferred from partial sequence data of rRNA genes; no further sequence data of S. mekongi is available yet. Knowledge of the molecular structure of protein coding genes of S. mekongi will provide a better understanding of gene function in the genus Schistosoma. A cDNA library of S. mekongi adult male was constructed and a cDNA encoding the 26 kDa glutathione S-transferase protein of this species was cloned. Sequence analysis of this cDNA confirmed the close phylogenetic relationship of S. mekongi to S. japonicum.