The whitebacked planthopper Sogatella furcifera is a serious pest of rice that has developed fipronil resistance in Asia. We found a novel R340Q mutation in the cytoplasmic loop between M3 and M4 of the S. furcifera RDL γ-aminobutyric acid (GABA) receptor subunit (SF-RDL). The R340Q mutation was found in clones carrying the A2′N mutation, which was previously implicated in fipronil resistance and suggested to be a heterozygous mutation. To investigate the influence of mutations, Drosophila Mel-2 cells were transfected with wild-type or mutant SF-RDL genes, either individually or together. A membrane potential assay showed that the influence of the A2′N·R340Q double mutation on fipronil resistance was more profound than that of the A2′N single mutation in the heterozygous expression of wild-type and mutant SF-RDL genes.