Antioxidant enzymes are thought to play a crucial role in the survival of the parasite, Schistosoma mansoni, during its migration through the tissues of the definitive host. We recently cloned the cDNA encoding one such enzyme, glutathione peroxidase (Gpx). In order to elucidate the regulation of expression of this gene, we describe the cloning and characterization of a Gpx gene of S. mansoni. An initial screen of a lambda EMBL4 genomic library using the corresponding cDNA sequence as a probe yielded 14 positive clones, two of which have so far been analyzed in detail. The complete Gpx gene contains five introns, four of which, located at the 5' end, are extremely short (30-51 bp) and the last of which is approximately 6 kb long. We present the sequence of the gene including 73 bp at the 5' end, the complete sequence to 137 bp downstream from the penultimate exon, 164 bp upstream and 131 bp downstream from the last 3' exon. The potential mRNA cap site is situated 219 bp upstream from the ATG start codon. All intron/exon junctions correspond to the conventional eukaryotic splice signal. Analysis of the 5' flanking region revealed the presence of a potential TATA box at--26 bp from the cap site, but no CAAT-like element is present. Southern blot analysis showed a unique Gpx gene organisation in the S. mansoni genome.