A two-year ®eld trial was conducted to study the effects of prolonged water stress on cassava (Manihot esculenta) productivity, and on nutrient uptake and use ef®ciency. Four contrasting cultivars were supplied with adequate fertilization and watering, except when water was excluded by covering the soil with plastic sheets for different periods, depending on treatment: from two to six months, four to eight months, or from six to twelve months after planting (early, mid-season and terminal stress respectively). Sequential harvests were made at 2, 4, 6, 8 and 12 months after planting to determine leaf area index and shoot and root biomass. At ®nal harvest, nitrogen, phosphorus, potassium, calcium and magnesium concentrations in shoots and storage roots were determined.During both early and mid-season stress, leaf area index and shoot and root biomass were signi®cantly smaller than those in the controls across all cultivars. After recovery from stress, leaf area index was greatly enhanced with less dry matter allocated to stems, and root yields approached those in the controls. One cultivar, CMC 40, had greater ®nal root yield under stress treatments. Nutrient concentration in roots and shoots was less in all cultivars with early stress and resulted in higher nutrient use ef®ciency in all elements for root production. The same trend was observed under mid-season stress, except for nitrogen concentration, which remained unchanged.Terminal stress did not affect leaf area index, but reduced the shoot biomass in all cultivars. Final root yields were smaller than those in the controls except for CMC 40 whose ®nal root yield was greater under stress. Nitrogen concentration was greater in root biomass but less in shoot biomass of all cultivars, resulting in lower nitrogen±use ef®ciency for root production. Across cultivars, only potassium-and magnesium-use ef®ciencies were greater than in the controls. CMC 40 was the only cultivar with consistently greater use ef®ciency of nitrogen, phosphorus, potassium, calcium and magnesium for root production under terminal stress. This higher nutrient use ef®ciency was due, mainly, to a greater root production rather than to smaller nutrient concentration. This cultivar is suitable as a gene source for improving cassava in order to maximize root pro-duction per unit nutrient extracted under stressful environmental conditions. introdution Throughout the cassava (Manihot esculenta)-growing areas of South America, the crop is an important staple food used mainly as a source of carbohydrates for