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The study aimed at determining the root dynamics of cactus pear following a soil-water gradient and to characterize some root types of the species O. ficus-indica (L.) Miller (cultivar Morado-green cladode) and O. robusta Wendl. (cultivar Monterey-blue cladode). One-year-old plants, planted in pots (210mm diameter and 550mm deep soil) and grown in the greenhouse at day/night temperatures of 25-30/15-18 degrees C, were investigated. The water treatments applied were 0-25%, 25-50%, 50-75%, and 75-100% depletion of plant available soil water. The impact of water stress was quandtified in terms of root and cladode mass, root length, water-use efficiency (WUE: dry weight gain for roots and/or cladodes per unit water used-evapotranspiration), and water needed to fill up a cladode. Root mass decreased and root length increased significantly for both species with water stress. The root mass and length of O. ficus-indica were for all water treatments higher (p <0.05) than that of O. robusta. By contrast, the root length/root mass ratio was higher4 for O. robusta than O. ficus-indica. Roots of O. ficus-indica and O. robusta, on average for all water treatments, comprised only 8.6% and 8.9% of the total plant biomass (dry weight), respectively. Root WUE decreased significantly (p <0.05) with water stress for both species. A water application of only 12 to 14mm is needed to fill up the cladodes of the two Opuntia species with water stress. Given water shortages on arid lands and climate change, more development attention needs to be given to Opuntia species.