The tolerance of Opuntia bigelovii Engelm. (Cactaceae) to high temperature was investigated by subjecting stems to temperatures ranging from 25Β°C to 65Β°C for a 1-h period, after which various properties of chlorenchyma cells were examined. The temperatures at which activities depending on membrane integrity decreased by 50% were 60Β°C for electrolyte leakage, 52Β°C for staining by neutral red, and 51Β°C for plasmolysis for plants maintained at day/night air temperatures of 30Β°C/20Β°C. Nocturnal acid accumulation, which depends on stomatal opening and enzymatic reactions as well as membrane properties, was half-inactivated at a lower temperature, 46Β°C. Visual observation indicated that 50% of the stems subjected to a heat treatment of 52Β°C became necrotic in 2 weeks.Heat acclimation, which is apparently necessary for survival of O. bigelovii in the field, was investigated by raising the day/night air temperatures from 12Β°C/2Β°C to 60Β°C/50Β°C in 10Β°C steps every 2 weeks. The heat tolerance of the cellular properties increased with increasing air temperature; for a 10Β°C temperature increase, the half-inactivation temperature increased 2.9Β°C for electrolyte leakage, 3.0Β°C for staining, 3.8Β° C for stem survival, and fully 6.1Β°C for nocturnal acid accumulation. The relative order of these four properties with respect to heat tolerance did not change during the hardening, nocturnal acid accumulation remaining the most heat sensitive. The upper temperature for 50% survival was 59Β° for O. bigelovii when acclimated to day/night air temperatures of 50Β°C/40Β°C.