Endocrine-disrupting chemicals (EDCs) have been shown to disrupt the reproduction of natural populations of several aquatic organisms including fish. Previously, we showed that some EDCs modulated the expression of oncogenes and tumor suppression gene, p53 in the hermaphroditic killifish Kryptolebias marmoratus. Exposure to EDCs may affect reproductive tissues including ovotestis of K. marmoratus not only through the estrogenic action but also by oxidative stress. Glutathione S-transferases (GSTs) provide antioxidant defense against oxidative stress in all the vital tissues. In this study, firstly, the full sequence of a cDNA clone of GST-Mu (GSTM) was identified by molecular cloning. The expression of GSTM was studied at different stages of development, and in the gonads of adult hermaphrodites and secondary males. Secondary males showed significantly lower expression of GSTM compared to hermaphrodites. When gonadal expression of GSTM was compared with other GSTs in the hermaphrodites, expression of GSTM was significantly higher than the others. Juvenile fish were exposed to three EDCs, bisphenol A (BPA), nonylphenol (NP) and octylphenol (OP) for 24h and expression GSTM transcript was studied at different time intervals. BPA at 12 and 24h caused upregulation of GSTM whereas OP showed similar effect at 12h only. NP, on the other hand, caused significant downregulation at 12 as well as 24h. When adult fish of both the genders were exposed to EDCs, except NP, the other EDCs caused significant upregulation of GSTM mRNA expression in gonads. Expression of GSTM in the gonads of secondary males was more pronounced than in the hermaphrodites. These findings indicate that the gonad of K. marmoratus has a robust antioxidant system. The gender-related differences in GSTM expression imply a differential sensitivity to EDCs in gender type.