Through use of a recently developed technique that can measure CO2 exchange by individual attached roots, the influences of soil O2 and CO2 concentrations on root respiration were determined for two species of shallow-rooted cacti that typically occur in porous, well-drained soils. Although soil O2 concentrations in the rooting zone in the field were indistinguishable from that in the ambient air (21% by volume), the CO2 concentrations 10 cm below the soil surface averaged 540 μLL−1 for the barrel cactusFerocactus acanthodes under dry conditions and 2400 μLL−1 under wet conditions in a loamy sand. For the widely cultivated platyopuntiaOpuntia ficus-indica in a sandy clay loam, the CO2 concentration at 10 cm averaged 1080 μLL−1 under dry conditions and 4170 μLL−1 under wet conditions. For both species, the respiration rate in the laboratory was zero at 0% O2 and increased to its maximum value at 5% O2 for rain roots (roots induced by watering) and 16% O2 for established roots. Established roots ofO. ficus-indica were slightly more tolerant of elevated CO2 than were those ofF. acanthodes, 5000 μLL−1 inhibiting respiration by 35% and 46%, respectively. For both species, root respiration was reduced to zero at 20,000 μLL−1 (2%) CO2. In contrast to the reversible effects of 0% O2, inhibition by 2% CO2 was irreversible and led to the death of cortical cells in established roots in 6 h. Although the restriction of various cacti and other CAM plants to porous soils has generally been attributed to their requirement for high O2 concentrations, the present results indicate that susceptibility of root respiration to elevated soil CO2 concentrations may be more important.