Micromelalopha troglodyta (Graeser) (Lepidoptera: Notodontidae) is an important pest of poplar trees, Populus spp. (family Salicaceae), in China. Herein, we report the first cloning and sequencing of a full-length cDNA of a delta class (Class I) glutathione S-transferase (GST), MtGSTd1, from the larval midgut of M. troglodyta. The open reading frame of the MtGSTd1 cDNA was 657 bp and encoded 219 amino acid residues in M. troglodyta. Furthermore, specific activities of GSTs were induced in fat bodies and midguts of M. troglodyta by tannic acid. MtGSTd1 mRNA also was induced by tannic acid in the fat bodies and midguts. GST activities increased following the elevated expression of GST mRNA in M. troglodyta. These findings indicate that the GSTs may have an antioxidant role in the metabolism of plant secondary substances in M. troglodyta larvae.