The subdominant CAM species, Echinocereus viridiflorus and Mammillaria vivipara, collected from the shortgrass prairie in northeastern Colorado were pretreated and analyzed for gas exchange under cool temperatures (20/15°C) and warm temperatures (35/15°C). Well watered plants of both species under a 35/15°C thermoperiod fixed atmospheric CO2 during the night and early moring. Echinocereus viridiflorus grown and analyzed at 20/15°C fixed CO2 during the night, early morning and late afternoon but total carbon gain over a 24 h period is less than when grown and analyzed under the 35/15°C thermoperiod. Mammillaria vivipara grown and analyzed at 20/15°C assimilates CO2 at low rates during all parts of a 24 h period with the greatest CO2 fixation rates occuring from midday to late afternoon. The total carbon gain under the 20/15°C thermoperiod is less than that for this species under the 35/15°C thermoperiod. Decreasing the night temperature of plants grown under the warm conditions to 10°C or 5°C results in a depression of the night CO2 fixation in both species. E. viridiflorus from the cool growth conditions showed an enhancement of the CO2 uptake during the night, early morning and late afternoon when subjected to the cooler night temperatures (10°C and 5°C). The CO2 uptake of M. vivipara grown at 20/15°C shows an enhancement during the night and early morning while the CO2 fixation during midday and late afternoon is slightly depressed under cool night temperatures (10° and 5°C). Under the 35/15°C thermoperiod both species exhibit depressed rates of CO2 fixation during the night and early morning when water stressed. Plants of both species grown under the 20/15°C thermoperiod exhibit no net CO2 fixation following five weeks of water deprivation. Upon rewatering, E. viridiflorus begins to recover its capacity for CO2 fixation within 24 h under both the warm and cool temperature regimes. However, M. vivipara did not show recovery within 48 h following rewatering under the warm or cool temperature regime. Contrasting the patterns of gas exchange of the subdominant species, E. viridiflorus and M. vivipara, with a dominant CAM species of the shortgrass prairie, Opuntia polyacantha reveals significant differences that may well dictate the role of these species in this ecosystem. E. viridiflorus and M. vivipara have a lower capacity of carbon gain and recovery from water stress than O. polyacantha mainly due to their lack of late afternoon CO2 uptake. This study suggests that carbon gain plays an important role in limiting E. viridiflorus and M. vivipara in the shortgrass prairie ecosystem.