The activity of photosynthesis and mitochondrial respiration, nocturnal organic acid accumulation and water relations were investigated in Prenia sladeniana L. Bol. [malic enzyme (ME)-type] andCrassula lycopodioides Lam. [phosphoenolpyruvate carboxykinase (PEPCK)-type] to compare the physiological responses to water deficit in crassulacean acid metabolism (CAM) plants differing in their decarboxylating enzyme systems. Withholding water inhibited daytime gas exchange within 2d while night time CO2gain and malic acid accumulation remained relatively unchanged in both species. In P. sladeniana, maximum photochemical efficiency (Fv/Fm) and photosynthetic electron transport declined to nearly the same degree as CO2supply was restricted during drought. Despite limited CO2availability, photosynthetic activity was largely unaffected in C. lycopodioides, as were mitochondrial properties. There is no indication of a drought-induced increase in the capability to totally oxidize malate, yielding 4 CO2, in either species. Nevertheless, the enhanced ratio of malate to glycine oxidation may have increased the in vivo capability for malate oxidation in P. sladeniana. Although pressure potential was maintained throughout the experiment in both species, active osmotic adaptation occurred only inP. sladeniana. The observed decrease in photosynthetic and mitochondrial activity may have resulted from the large increase in osmotic concentration in this species.