Hylocereus undatus (Haworth) Britton and Rose growing in controlled environment chambers at 370 and 740 μmol CO₂ mol⁻¹ air showed a Crassulacean acid metabolism (CAM) pattern of CO₂ uptake, with 34% more total daily CO₂ uptake under the doubled CO₂ concentration and most of the increase occurring in the late afternoon. For both CO₂ concentrations, 90% of the maximal daily CO₂ uptake occurred at a total daily photosynthetic photon flux density (PPFD) of only 10 mol m⁻² day⁻¹ and the best day/night air temperatures were 25/15°C. Enhancement of the daily net CO₂ uptake by doubling the CO₂ concentration was greater under the highest PPFD (30 mol m⁻² day⁻¹) and extreme day/night air temperatures (15/5 and 45/35°C). After 24 days of drought, daily CO₂ uptake under 370 μmol CO₂ mol⁻¹ was 25% of that under 740 μmol CO₂ mol⁻¹. The ratio of variable to maximal chlorophyll fluorescence (F_y/F_m) decreased as the PPFD was raised above 5 mol m⁻² day⁻¹, at extreme day/night temperatures and during drought, suggesting that stress occurred under these conditions. F_v/F_m was higher under the doubled CO₂ concentration, indicating that the current CO₂ concentration was apparently limiting for photosynthesis. Thus net CO₂ uptake by the shade‐tolerant H. undatus, the photosynthetic efficiency of which was greatest at low PPFDs. showed a positive response to doubling the CO₂ concentration, especially under stressful environmental conditions.