Stem rust, caused by Puccinia graminis f. sp. tritici Ericks and E. Henn., is an important disease of wheat (Triticum aestivum L.) in many countries. This study assessed the impact of race TTKS (Ug99) of stem rust on grain yield and kernel weight. Three replicated trials including six commercial wheat varieties were sown in three sites for 2-years. Final disease severities (FDS %) observed in the untreated plots averaged 44.4%, 43.3% and 56.3% in 2006 and 28.7%, 34.7% and 38.1% in 2007 at KARI-Njoro, Mau-Narok and Eldoret, respectively. Similarly, the area under disease progress curves (AUDPC) were on average high in the untreated plots across the sites compared with the treated, indicating that all the varieties were susceptible to race TTKS (Ug99) of stem rust. The mean grain yield was on average significantly lower for untreated plots (0.99, 2.18 and 1.09 t/ha) compared with the fungicide-treated (1.75, 3.75 and 2.03 t/ha) at KARI-Njoro, Mau-Narok and Eldoret, respectively, over the two years. Kernel weight was reduced by 27.2%, 22.7% and 76.7% in 2006, and in 2007 the loss was 44.6%, 8.5% and 22.5% at KARI-Njoro, Mau-Narok and Eldoret, respectively. These results demonstrated that all the commercial wheat varieties tested were susceptible to race TTKS (Ug99) of stem rust, and suffered high grain yield reduction. Even though fungicide treatments increased grain yields and kernel weight appreciably, the treated plots were not completely rust-free, suggesting further investigation for a more appropriate and effective fungicide for the reduction of stem rust....