Technical Abstract: Modern spinning techniques and new end uses have created demands for improved fiber quality in upland cotton. However, genetic variation available for fiber improvement is restricted in commercial upland cotton (Gossypium hirsutum L.), and occurs in genotypes that exhibit high levels of heat sensitivity. Three lines (AGC85, AGC208, and AGC375) possessing superior fiber length and strength characteristics and improved yield performance under heat stress environments have been developed by the USDA-ARS and Cotton Incorporated and released in 2005. All three germplasm lines originate from the cross of the commercial cultivars 'FiberMax958' and 'SG248'. Individual plant selection within early generation progeny (F2 and F3) was practiced under heat stress conditions at the low desert location of Maricopa, AZ in 2001 and 2002. Progeny testing was conducted in 2003 and 2004 at Maricopa, AZ and Tifton, GA, and Shafter, CA. Averaged over locations, the lint yields of AGC85 (2021 kg ha-1) and AGC375 (1874 kg ha-1) were equivalent to that of SG747 (1938 kg ha-1), a high yield performance check. The fiber lengths (UHM) of AGC85, AGC208 and AGC375 were greater than or equal to the high fiber quality parent FiberMax958 (28.7mm). The fiber length of AGC208 was superior to that of the Acala cultivar, Phytogen 72 (29.5mm). Fiber bundle strengths of AGC85, AGC208 and AGC375 were stronger than the FiberMax 958 parent (31.4cN). Two lines, AGC85 and AGC375, exhibited fiber strength equivalent to that of Phytogen 72 (34.0cN). The lines have been made available to public and private breeders as resources for concurrent improvement of fiber quality and heat tolerance in upland cotton.