To identify the chromosome carrying the factor for resistance to M. incognita in tobacco, crosses were made between resistant tobacco ''NC95'' as pollen parent and each of the 12 tobacco monosomics (A-L) representative of the Tomentosae half of the N. tabacum chromosome complement. Of the F1 seedlings, 927 plants were grown for observation. From these, 223 plants were selected as possible monosomics on the basis of morphological characteristics. These plants were self-pollinated, and the resulting F2 plants were inoculated with both M. incognita acrita and M. incognita incognita. Sixteen F2 populations, derived from the haplo-G monosome, were completely resistant. All of the F2 populations derived from the other 11 monosomic crosses segregated into a 3:1 (resistant:susceptible) ratio. The factor for resistance to M. incognita is located on the G chromosome of N. tabacum. This is the 1st report establishing the N. tabacum chromosome that carries the factor for root-knot resistance. The results are consistent with earlier evidence that M. incognita resistance in tobacco is derived from N. tomentosa, a species in the section Tomentosae of the subgenus Tabacum, genus Nicotiana. The other 12 chromosomes of N. tabacum have affinities with N. sylvestris, section Alatae, subgenus Petunoides, genus Nicotiana.