Resistance to cotton-melon aphid (Aphis gossypii Glover) segregated as a single dominant gene in a melon (Cucumis melo L.) mapping population derived from the cross `Top Mark' × PI 414723. Sixty-four F2-derived F3 families were used to map the aphid resistance locus, Vat, with respect to randomly amplified polymorphic DNA (RAPD) and restriction fragment length polymorphism (RFLP) markers. RFLP markers NBS-2 and AC-39 flanked Vat at distances of 3.1 cM and 6.4 cM, respectively. NBS-2 is homologous to the nucleotide binding site-leucine-rich repeat (NBS-LRR) superfamily of plant resistance genes. Another homolog of this superfamily, NBS-5, was positioned ≈16.8 cM from Vat, raising the possibility that Vat resides in a cluster of NBS-LRR paralogs. RFLP marker AC-8, which has similarity to plant lipoxygenases, was positioned at ≈5.5 cM from Vat. Monogenic resistance to A. gossypii has been identified in two sources of melon germplasm, Indian accession PI 371795 (progenitor of PI 414723) and Korean accession PI 161375. To test for an allelic relation between the genes controlling aphid resistance in these two distinct germplasm sources, melon plants of a backcross population from a cross between two resistant lines having Indian- or Korean-derived resistance were infested with aphids. At least 90 out of 92 segregating progeny were aphid resistant, suggesting that the same resistance gene, Vat, is present in both sources of melon germplasm.