The response of cassava (Manihot esculenta Crantz) to water stress was studied in two distinct sites (Tucson, Arizona and Santander de Quilichao, Colombia). A third experiment was conducted in Palmira, Colombia to evaluate the relationship between photosynthesis, relative humidity and yield of cassava cultivars HCol 1684. Plants stressed after 2 months of growth in Tucson reduced growth by reducing leaf formation, expansion and leaf area. Reductions in LAI due to stress imposed to MCol 1684 in Quilichao were attained by reducing leaf expansion rates and leaf formation in plants stressed after 3 months of age. Plants stressed later increased leaf fall and did not reduce leaf formation. Stress reduced dry matter production in all cultivars, especially when given to young plants. Stress also altered the patterns of dry matter partitioning in 3-month-old plants of MCol 1684, but not in those stressed at 6 months. Transpiration and diffusive conductances of MCol 1684 were reduced after 40 days of stress. These parameters were correlated to photosynthesis and leaf temperatures for non-stressed plants, and additionally with relative humidity in the plants stressed after 3 months of growth. Interestingly, leaf temperatures were lower in stressed plants of MCol 1684, which suggests that stressed cassava plants can avoid excessive heating caused by stomatal closure simply changing leaf orientation and increasing reflectance. Leaf water potential was slightly reduced by stress in Tucson except for MVen 218. Stressed plants of MCol 1684 in Quilichao showed lower values of (L) than those of non-stressed plants after 30-40 days of treatment, suggesting an adaptation of stressed plants acquired during the stress period. The effects of air humidity on stomatal functioning of MCol 1684 seems to be strong as suggested by the dependence of transpiration, conductances and photosynthesis on relative humidity.