In soybean [Glycine max (L.) Merrill], there are 30 independently-derived necrotic root mutant lines. Necrotic root is a disease lesion mimic mutation, which mimics the phenotype that results from Phytophthora sojae infection of soybean seedlings. The necrotic root mutants are conditional lethals. One mutant line arose from chemical mutagenesis, two spontaneously (?) from segregating populations, and 27 from a gene-tagging study involving the w4-mutable line. Our objectives were to; 1) determine allelism of the 30 mutant lines, and 2) molecularly map the necrotic root locus or loci. Allelism tests were done by crossing normal plants as female parents. Plant identity was maintained and only heterozygous plants were used in the testcrosses. Homozygous necrotic root plants were male parents and were the result of grafting a necrotic root scion onto a normal rootstock. All 30 necrotic root mutants were allelic based upon testcross and F2 data. SSR markers were used for molecular mapping in the cross of Minsoy x homozygous necrotic root plants. The necrotic root locus, rn, was located between Satt 288 and Satt 612 on Molecular Linkage Group G.