Yield losses due to powdery mildew infection for the barley culti­vars Giza 123, Giza 124, Giza 125, and Giza 126 were determined dur­ing two consecutive seasons, 1994/95 and 1995/96 at Giza and Gemmeiza, respectively. The single tiller technique was used. Yield com­ponents, 1 000-kernel weight (1000-KWT) and weight of grains per head were determined. The results indicated that gradual reduction in 1000 KWT and weight of grains per headregularly correlated with the increase in mildew severity to 40%. A relatively slight reduction in 1000-KWT due to mildew infection was noticed in Giza 123 compared with the other cultivars. Grain yield losses was also observed and was amounted to about 18%.