Glutathione peroxidase is an important enzyme in the endogenous antioxidant defense system. In the current study, we cloned the full-length cDNA sequence that encodes a glutathione peroxidase from Magallana hongkongensis (designated as MhGPx), investigated the tissue expression profile of MhGPx mRNA, and studied the time-dependent effects of cadmium exposure and air exposure at two temperatures (4 °C and 15 °C) on the mRNA level of MhGPx in different tissues (including gill, adductor, mantle, and digestive gland). The full length of MhGPx was 1306 bp. This gene was found to contain a selenocysteine insertion sequence within the 3′ untranslated region, which indicates that the protein it encodes was a selenoprotein. In addition, MhGPx was constitutively expressed in all four selected tissues, and the level of expression in the digestive gland was highest, indicating that MhGPx mRNA expression has tissue-specificity. Under cadmium exposure, mRNA expression was significantly increased on day 1. When exposed to air, the highest MhGPx mRNA expression in the 4 °C group was lower and appeared later than that of the 15 °C group. The results of this study contribute to a better understanding of oyster GPx in response to oxidative stress. [ABSTRACT FROM AUTHOR]