Root respiration, measured as CO 2 efflux, was studied for a succulent perennial from the Sonoran Desert, Agave deserti, with a new technique using individual, attached roots. The daily patterns of root respiration closely followed the daily patterns of root temperature for both established roots and rain roots, with higher rates during the day when root temperature averaged 27°C and lower rates at night when root temperature averaged 17°C. When root temperature was raised from 5°C to 40°C, root respiration increased about 7-fold; from 45 °C to 55 °C, root respiration decreased about 2-fold, except for old established roots. Root respiration per unit dry weight for both root types decreased with age, the initial decrease being greater for rain roots than for established roots. Root respiration rates for rain roots were reduced to zero at a soil water potential (ψ soil) of -0.9 MPa and did not recover upon rewatering. Upon drying, root respiration rates for established roots were maintained at about 12% of maximum, even when ψ soil fell to -1.6 MPa, and fully recovered 1.5 d after rewatering the soil. Such responses of rain and established roots must be taken into account when assessing the carbon costs for the root system.