A full-length cDNA, with an open reading frame (ORF) of 1,449 bp, encoding a subunit of the gamma-aminobutyric acid (GABA)-activated chloride channel was isolated from Plutella xylostella (L.) (Lepidoptera: Plutellidae) (GenBank accession no. EF156251). The subunit gene encoded a 483-amino acid polypeptide that showed 84% sequence identity with DmRdl subunit (U02042) (Drosophila melanogaster resistant to dieldrin). When expressed in Xenopus laevis oocytes, the subunit assembled as a functional homomeric complex activated by GABA and abamectin in a dose-dependent manner. The EC50 value of GABA was 0.49 mM (0.41-0.58) (n = 5). However, the responses to abamectin were very robust, with an EC50 of 4.85 microM (4.02-5.89) (n = 6), indicating that abamectin was > 100-fold more potent in activating chloride currents than GABA. The results suggest that this subunit is vital to the formation of a functional channel and contains the binding site of abamectin.