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1. Root elongation as a function of soil temperature was determined for the CAM succulent Opuntia ficus-indica, under three different day/night air temperatures (15 °C/5 °C, 25 °C/15 °C and 35 °C/25 °C) and an ambient (360 μmol mol–1) vs a doubled CO2 concentration (720 μmol mol–1) at 25 °C/15 °C, the optimum temperature for net CO2 uptake. 2. Root elongation occurred at soil temperatures from 12 °C (at 15 °C/5 °C) to 43 °C (at 35 °C/25 °C) with optimum temperatures of 27–30 °C, similar to other CAM succulents and consistent with the distribution of this shallow-rooted species in warm regions. Although a doubled CO2 concentration did not alter the optimum or limiting soil temperatures, increases of up to 5 °C in these temperatures accompanied the 20 °C increase in day/night air temperatures. 3. Root elongation rates at optimum soil temperatures ranged from 5·4 mm day–1 (15 °C/5 °C), through 6·6 mm day–1 (25 °C/15 °C), to 10·4 mm day–1 (35 °C/25 °C) with a 25% increase under a doubled CO2 concentration. Highest root elongation rates at 35 °C/25 °C may reflect changing root vs shoot sink strengths in a species with a highly plastic root system. 4. At limiting soil temperatures, the length of the cell division zone was reduced by an average of 20% and cell length at the mid-point of the elongation zone by 10%. Increased root elongation rates under a doubled CO2 concentration reflected increased cell elongation. 5. The temperature response for the roots of O. ficus-indica and stimulation of elongation by a doubled CO2 concentration indicate that root growth for this highly productive species should be enhanced by predicted global climate change.