This work analysed the selenoprotein glutathione peroxidase (GPx) activity in eggs of Sparus aurata. GPx was found in eggs before and after fertilization, in embryos during development, at hatching and during larval development. The content was very low in segmentation and organogenesis whereas increased at the end of embryogenesis and in larval development. This study provides evidence on how selenoproteins as GPx exert an antioxidant defense in sea bass development and sustain antioxidants as quality index.