Lint yield of cotton, particularly Pima (Gossypium barbadense) can be reduced by high temperatures during peak flowering periods in irrigated Southwestern conditions. High stomatal conductance (G_S) may confer adaptive advantage to genotypes that experience supra-optimum temperatures by its association with elevated leaf cooling to reduce canopy temperatures (1). Interspecific differences for G_S has been well established. The objective of this research was to practice divergent selection for G_S in a population derived from G. hirsutum X G. barbadense introgression (TM1 X NM24016). TM1 is a typical G. hirsutum and NM24016 (2) is a introgressed line derived from G. hirsutum X G. barbadense hybridization program. Divergent selection for G_S was practiced on the F_2.3 generation in Maricopa, AZ in 1996. DNA was isolated from all 118 F₂ plants in 1995 for genetic mapping experiments and QTL analysis of G_S. Replicated field experiments of selected F_2.4 progeny (10 high G_S and 10 low G_S) were grown in Maricopa, AZ and Las Cruces, NM in 1997. G_S was measured at both locations at peak flowering as described by Radin et al. (3). Based on F₃ and F₄ data the realized H for G_S was estimated to be 0.41 in Maricopa. This is reflected in the significant difference between the mean of the high GS F_2.4 lines (n=10) 552 mmol m^-2 s^-1 and the low G_S lines (n=10) 457 mmol m^-2 s^-1 at Maricopa in 1997. The difference at Las Cruces was non-significant (314 for high G_S vs 282 for low G_S). The absence of supraoptimum temperature at Las Cruces relative to Maricopa probably explains this difference. Correlated response in lint yield was observed in Maricopa but not Las Cruces. The high G_S lines were significantly higher yielding than the low G_S lines at Maricopa even though the only selection has been for G_S. Composite Interval Analysis was conducted on the F_2.3 progeny in Maricopa utilizing DNA markers (RAPDs and SSRs). Two QTLs (LOD = 2.0 and 3.8) were detected for G_S at Maricopa. These QTLs explained about 12% of the variation in the trait. Both intervals were from NM24016 and negatively affected G_S. It is suspected, but not yet verified, that these regions are derived from G. barbadense. This is one of the first physiological traits that have been subjected to QTL analysis. G_S is a heritable trait and seems to be significantly associated with lint yield in heat stress environments.