Carbon dioxide exchange patterns of prickly-pear cactus (Opuntia phaeacantha var. discata) were studied throughout the year to determine temperature influences on and seasonal responses of the process. Cacti exhibit CAM which permits nighttime carboxylation of CO2 to malate and daytime decarboxylation of malate to CO2. The gas exchange studies were done on plants harvested near Mesa, Arizona, and placed in an open CO2 exchange analysis system. Carbon dioxide exchange rates varied with temperature and season. Greatest CO2 influx rates were at low temperatures while efflux rates were greatest at high temperatures. A shift in season caused a change in CO2 exchange rates at any one temperature. Equal rates were shown at increasing temperatures as CO2 analyses progressed from winter to summer showing seasonal temperature acclimation. A comparison of the CO2 exchange temperature compensation point (the temperature at which the plant shows zero CO2 exchange usually due to the shift from net influx to net efflux or the opposite) with air temperatures shows the months of November through February to have the greatest potential for a large net CO2 influx while the summer months may actually exhibit a small net CO2 loss. This loss is much less than would occur if the cacti did not exhibit temperature acclimation. Thus, with a large net CO2 influx in the cool months and a small net CO2 efflux during the warm months, prickly-pear cacti maintain a net CO2 exchange gain for the year.