Glutathione S-transferase (GST) is one of the most important phase II drug-metabolizing enzymes, catalyzing the conjugation of electrophilic substrates to glutathione. Unlike in humans, a surprisingly limited number of GST genes have been identified in monkeys. The identification of additional GST genes in this model system would prove to be advantageous, because monkeys remain an important predictor of drug effects and toxicities in humans during preclinical studies. In this study, we report the identification and characterization of the following six cDNAs in cynomolgus monkeys: mfGSTA1, mfGSTA2, mfGSTM5, mfMGST1, mfGSTO1, and mfGSTZ1. These cDNAs encode GSTs highly homologous (approximately 95%) to human GST cDNAs. Among these, the mfGSTA1, mfGSTM5, mfMGST1, mfGSTO1, and mfGSTZ1 cDNAs correspond to a single human GST counterpart, whereas the mfGSTA2 cDNA is highly similar to human GSTA1 and GSTA2 cDNAs. An analysis of tissue samples indicates that these GST genes are predominantly expressed in the liver along with some extrahepatic expression as determined by real-time reverse transcriptase-polymerase chain reaction. It is interesting to note that mfGSTA2 is significantly differentially expressed between males and females in the jejunum, where a striking 8-fold higher expression level is observed in males. These results suggest that a potential sex difference in the metabolism of drugs may be mediated by mfGSTA2. This also provides a basis for the investigation of sex-dependent drug metabolism in monkeys.