In vertebrates, both reactive oxygen species (ROS) and the corresponding scavenging system components especially glutathione peroxidase (GPx) are indispensible for normal development of the gonads. To investigate the function of GPx in crustaceans, we cloned and characterized a full length GPx (MeGPx) transcript in the penaeid shrimp Metapenaeus ensis. Phylogenetic analysis showed that MeGPx clustered with the GPx from mollusks and nematodes and shared much higher similarity with vertebrate GPx3 and GPx5 than with GPx1 or GPx2. Multiple sequence alignment further demonstrated that MeGPx is evolutionarily conserved among invertebrates, with common functionally important motifs. MeGPx was specifically expressed in shrimp ovaries, but not in other tissues studied, including testis. In situ hybridization showed that MeGPx was highly expressed in pre-vitellogenic and mid-vitellogenic oocytes, while no expression was detected in late-vitellogenic oocytes. Moreover, real time PCR showed high level expression of MeGPx in the early ovaries. Since active protein synthesis and deposition occurred in mid-vitellogenic oocytes, MeGPx might play a pivotal role in preventing oocytes from oxidative damage and balancing ROS production. The present findings on shrimp GPx provide insights on the regulation of ROS in the ovarian maturation process and the role of GPx in crustacean reproductive biology.