A Schistosoma mansoni cDNA library was constructed from the mRNA of adult worms in the expression vector lambda gt11 and screened with a rabbit antiserum raised against the 26-kDa S. mansoni glutathione S-transferase isoforms (Sm GST 26). Two clones were selected and the nucleotide sequences deduced. The predicted amino acid sequence, specified by these cDNAs, shows strong homology with a Schistosoma japonicum 26 kDa glutathione S-transferase and a lower level of homology with mammalian glutathione S-transferase class mu isoenzymes (EC 2.5.1.18). No significant homology score was found with a 28-kDa S. mansoni glutathione S-transferase (Sm GST 28). A study of the tissue distribution of the cloned Sm GST 26 by immunoelectron microscopy shows similarities to Sm GST 28 in that they are present in the tegument and in subtegumentary parenchymal cells. However, a major difference exists in the protonephridial region in which Sm GST 26 is present in the cytoplasmic digitations localized in the apical chamber delineated by the flame cell body, suggesting that Sm GST 26 may be actively excreted by adult worms.