The race Ug99 of Puccinia graminis f. sp. tritici causing stem rust disease of wheat was initially identified in Uganda in 1998. It was designated as TTKSK based on the North American nomenclature and has caused periodic losses to wheat crops in East Africa. Ug99 has recently moved out of Africa to Yemen and West Asia. The most effective approach to prevent losses from stem rust is through the deployment of resistant cultivars. More effective sources of resistance need to be identified and incorporated in the existing commercial cultivars. The first Stem Rust Resistance Screening Nursery (1stSRRSN) assembled by the International Maize and Wheat Improvement Center (CIMMYT) consisted of elite and advanced CIMMYT bread wheat lines and was evaluated for resistance to Ug99 in Njoro, Kenya for four consecutive seasons (2005 to 2007). Seedling reactions were determined in the greenhouse at the Cereal Disease Laboratory, St. Paul, MN. Two race-specific genes, Sr24 and Sr25, were found to confer resistance to Ug99, although Sr24 became ineffective to a mutant race of Ug99, TTKST, in 2007. Three previously uncharacterized genes, one each from synthetic wheat, Chinese germplasm, and other genetic backgrounds, were detected. Although 30% of the screened lines were susceptible in the seedling stage, these lines displayed various levels of adult plant resistance (APR) in the field tests. Presence of the APR gene Sr2, identified based on the pseudo-black chaff phenotype on glumes and darkened internode, was common in wheat lines with APR. The information on the resistance identified in the 1stSRRSN constitutes an important source for breeding wheat for durable resistance.