Interest in developing the guayule plant ( Parthenium argentatum Gray) as a source of natural rubber has increased in the United States and other countries during the last decade. A comprehensive study was initiated to obtain information on the production and water-use relationships for guayule. Water-use efficiencies for three cultivars after 2 years ranged from 0.70 to 0.85 kg/m 3 for dry matter, 0.045 to 0.055 kg/m 3 for resin, and 0.03 to 0.04 kg/m 3 for rubber production. The dry matter, resin, and rubber yield were shown to be linearly related to evapotranspiration. Large quantities of irrigation water were needed to get high yields in this short period of time. Relating yield and crop stress on a seasonal basis indicated that guayule grown in an arid environment was more sensitive to water stress in the latter than the first half of the year. Although the drought tolerance characteristics of the guayule crop permits flexibility in irrigation scheduling, the irrigation water requirement can be high in order to increase yields and shorten the harvest time to 3 years or less.