Stripe rust of wheat caused by Puccinia striiformis f. sp. tritici reduced kernel mass and the number of kernels per head in field epidemics from 1984 to 1987 in southern New South Wales. These epidemics began at the stem elongation stage of growth in 1984 and at booting to heading stages in the other years. The effects were greatest in very susceptible cultivars when the epidemic began before the booting stage of growth and affected more leaf area by the early milk stage of growth. Grain yield was reduced by up to 84%, kernel mass by up to 43% and kernel number by up to 72%. Stripe rust did not affect plant height, tiller number or stem dry matter at booting and anthesis. In some cases, stem dry matter was reduced at maturity. The dates of booting, heading and anthesis were not affected. The possible implications of the effect of stripe rust on grain quality arising from reduced kernel size are discussed.