Glutathione S-transferase (28GST) with molecular mass of 28 kDa is an anti-oxidant enzyme abundant in Clonorchis sinensis. In adult C. sinensis, 28GST was localized in tegumental syncytium, cytons, parenchyma, and sperm tails examined by immunoelectron microscopy. C. sinensis 28GST was earlier found to neutralize bio-reactive compounds and to be rich in eggs. Accordingly. it is suggested that 28GST plays important roles in phase II defense system and physiological roles in worm fecundity of C. sinensis.