The effect of drought stress in isolation, or in combination with beet yellows virus infection, on sugar beet growth was studied in the field and glasshouse. Drought reduced total plant weight by 26%, due to 20 and 29% reductions in foliage and storage root yields respectively. Sugar extraction efficiency was depressed by an increase in amino-nitrogen impurities. Drought did not limit water extraction depth, despite decreasing lateral root growth in proportion to total weight. During the field experiments, total crop cover was decreased in all the droughted treatments (halved in some cases) for at least part of the season. Consequently, these treatments intercepted 12% less light, which in combination with a 16% decrease in the dry matter/light conversion coefficient, led to the decrease in growth. The decrease in conversion coefficient was due to temporary closure of the stomata rather than a function of drought-induced damage to the photosynthetic mechanism. The absolute effect of drought remained the same irrespective of whether the plants were infected with beet yellows virus, i.e. there was no interaction between the two stresses. The reasons for this lack of interaction are discussed but it is likely that the stress effects were mediated at different times of the day and season.