Utilization of germplasm from certain relatives of N. tabacum could be of value for improving quantitative traits in cultivated tobacco.  Also, introgression of germplasm from closely related species (those from sections Tomentosae and Sylvestres) might facilitate mapping of genes underlying quantitative traits.  To investigate these possibilities, we examined near isogenic tobacco lines (NILs) differing for an introgressed chromosome segment derived from N. tomentosa which carries a quantitative trait locus (QTL) that exhibits a large influence on leaf number and correlated traits.  Within a ‘Red Russian’ genetic background, the QTL was found to act in an additive to partially dominant fashion with respect to leaf number, flowering time, plant height, and green leaf yield.  The effect of the QTL in diverse genetic backgrounds was investigated through field experiments that evaluated F₁ hybrids between the two Red Russian NILs and each of six tobacco lines varying widely for plant type, leaf number, flowering time, and yield.  The introgressed QTL affected all of these traits, but to varying degrees in the different backgrounds.  The degree of recombination within the introgressed N. tomentosa segment was then investigated.  Fifteen AFLP markers were identified that differentiated the two NILs.  These were used to genotype 207 field-grown BC₁F₁ individuals segregating for the introgressed QTL.  All markers were found to be located within a single linkage group.  All BC₁F₁ plants were phenotyped for leaf number, flowering time, plant height, green leaf yield, leaf length, and leaf width.  Significant associations between all markers and measured traits were found.  Sufficient recombination within the region allowed for mapping the chromosome position of the alien gene(s) influencing the measured traits.  Introgression of germplasm from diploid species closely related to N. tabacum may contribute useful genetic variation and facilitate mapping of genes controlling quantitative traits in Nicotiana.