The fall armyworm (FAW),Spodoptera frugiperda(J.E. Smith; Lepidoptera: Noctuidae) is an invasive agricultural pest with a global distribution, causing major crop losses annually. Its control strategies largely rely on chemical insecticides and transgenic crops expressingBacillus thuringiensisinsecticidal proteins (Cry and Vip toxins); however, the development of high resistance poses a significant issue. The ATP-binding cassette transporter C2 (ABCC2) has been linked to Cry toxin pore formation, acting as a receptor of some Cry toxins. Recently detected mutations in the SfABCC2 gene in extracellular loop 4 (ECL4) have been associated with Bt toxin resistance in FAW. In the present study, we expressed the SfABCC2 gene inDrosophila melanogaster, a species normally unaffected by the Bt toxins. We demonstrate that susceptibility can be introduced by the ectopic and tissue-specific expression of wildtype SfABCC2. Next, we introduced mutations into ECL4—both individually and in combination—that have been recently described in Brazilian FAW and functionally validated by toxicity bioassays against the foliar Bt product Xentari. Our results provide an efficient demonstration of the suitability of transgenicDrosophilafor validating FAW ABCC2 resistance mutations in ECL4 against Bt toxins, and potential cross-resistance issues between closely related proteins that use ABCC2.