Two methods were used: 1. The effect of shrivel)ed grain on hectoliter weight (H.W.) arid yield, and the determination of yield decrease due fo a reduction of 1 kg in H.W., was studied in seven wheat variadas, 2. Determination of yield losses dueto decrease of H.W., consideringal) thevarieties under yield trials, from 1975to 1983, With the first method, the yield )osses due to a reduction of 1 kg in H.W. were, in average 3.50/o, with a range between 3.3% and 4.0%. With the second method, it was found that in the years 1980, 1981, and 1982, when there were no important problems affecting wheat, the decrease in yie)d dueto a lower H.W. was small or nil. From 1975 to 1979, when wheats were affected by severa) problems (virus, lodging, P. graminis),yield decrease was, in the average, 3% par 1 kg reduction in H.W. These losses, in four of the five years considerad, were very similar to the one obtained with the first method. This could be explained be- cause in those years losses in yield were mainly dueto the shrivelling of the grain. In 1983, the yield losses doe to reduction of 1 kg in H.W. were of 5.0% in average. These larga losses were due fo shriveliing plus reduction in grain numbers. It was concluded, that 1 kg reduction in H.W. reduces yield in 3,5%, it there is no decrease in grain numbers. When there is reduction in numbers of grain, the yield losses will be higher.