two field experiments were carried out at Carillanca Experimental Station (38º45'S Latitude, 72º24'W Longitude) during 1975/76 and 1976/77, to determine the natural infective potencial of Puccinia striiformis West., on five commercial winter wheat cultivars. Disease intensity, during the different phenological stages, was related to crop yield components. Yiel losses up to 60% were observed, according to the cultivar's susceptibility. Yield components statistically affected were: 1.000 kernels' weight, number of kernels per square meter, and average number of kernels per spike. The latter varied according to the development of stripe rust on each cultivar. Stripe rust infection affected the different cultivars from stage 7 to stage 10.5.4, in the Feekes-Large scale.