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Opuntia is a drought tolerant crop and even the smallest amount of water is absorbed efficiently through the shallow and horizontally spread root system. However, there is a lack of knowledge on the root dynamics of cactus pear for sustainable production of fodder and fruit in the drier areas. This study, conducted during the 2003/2004 growing season on two-year-old Opuntia ficus-indica (L.) Miller (cultivar Morado-green cladode) and O. robusta Wendel. (cultivar Monterey-blue cladode) plants in the field was therefore aimed at quantifying root distribution with distance and depth from the stem. Root growth was expressed in terms of both mass and length, and water-use efficiency (WUE) was defined as the cladode dry mass production per unit of evapotranspiration. In both species most roots were concentrated in the first 150 mm soil layer. After only two growing seasons the roots spread as far as 2.5 m from the stem for both species. The total root dry weight production calculated per plant up to a depth of 1200 mm, was 239 and 316 g per plant or kg ha- 1 for O. ficus-indica and O. robusta respectively. The thickest roots developed directly from the planted cladode and were 9.1 mm thick for O. ficus-indica and 6.3 mm for O. robusta. Opuntia robusta showed a finer root system than that of O. ficus-indica. After two growing seasons 3407 and 2702 kg ha- 1 aboveground dry mass was produced by O. ficus-indica and O. robusta respectively. The WUE of O. ficus-indica (6.52 kg ha- 1 mm- 1) was significantly higher than that of O. robusta (5.39 kg ha- 1 mm- 1). The roots comprised only 11% of the total biomass for O. robusta and 7% for O. ficus-indica. Such root adaptations of this species are in addition to the classical physiological and structural modifications of CAM plants to tolerate prolonged drought.