Glutathione S-transferases (GSTs) play a major role in detoxification of xenobiotics and antioxidant defense. Here we report full-length cDNA sequence of a novel Sigma-class of GST (GST-S) from the intertidal copepod Tigriopus japonicus. The full sequence was of 1,136 bp in length containing an open reading frame (ORF) of 651 bp that encoded 217 amino acid residues. The recombinant Tigriopus GST-S was highly expressed in transformed Escherichia coli. Kinetic properties and effects of pH, temperature and chemical inhibitors on Tigriopus GST-S were also studied. The expression of GST-S was studied using real-time RT-PCR in response to exposure to two oxidative stresses-inducing agents, viz., hydrogen peroxide (H(2)O(2)) and heavy metals (copper, manganese). It was observed that H(2)O(2) (2mM) exposure down-regulated its expression at the initial stage but there was recovery and up-regulation shortly afterwards. In case of heavy metal exposure there was concentration-dependent increase in Tigriopus GST-S gene expression up to 24h. These results suggest that Tigriopus GST-S expression is modulated by prooxidant chemicals and it may play a role against oxidative stress. A majority of other GST isoforms is known to play an important role in antioxidant defense. This study provides a preliminary insight into the possible antioxidant role for Sigma-class of GST in T. japonicus.