Reniform (Rotylenchulus reniformis) nematodes are pathogens of cotton (Gossypium hirsutum) and soybean (Glycine max.) in the southern Unites States. The nematode costs US cotton growers over $100M per year in lost yield, quality and management expenses. Extremely strong resistance was found in the wild African diploid species G. longicalyx and introgressed into tetraploid cotton. Two highly resistant cotton lines were released, LONREN-1 and LONREN-2, but field testing revealed severe stunting of seedlings grown in fields heavily infested with reniform nematodes. Possible explanations include root damage caused by hypersensitivity to the nematode, per se, and/or new or accentuated susceptibility to one or more soil-borne pathogens, e.g., due to loss of linked (repulsion) G. hirsutum disease resistance loci. Several nematode and fungal resistance loci of cotton are known to reside nearby. To better characterize this chromosomal region and potentially separate the resistance and stunting, we have mounted a map-based approach to recover recombinants that retain the alien resistance gene (Renlon) but minimize the flanking alien segments. We are using resistance-linked SSR markers BNL3279, BNL1231, BNL1066, and CIR316 to identify and differentiate among nearby recombinants. To exploit this strategy at high resolution, we are saturating this region by developing SNP markers for G. longicalyx, and localizing those in the alien segment region. The SSRs and SNPs will then be used on large populations to recover recombinants closely flanking the alien resistance gene, as well in nearby regions. The most closely flanking recombinants will be phenotyped for resistance, stunting and tested for agronomic performance.