Under ecologically realistic environmental conditions, the water-use efficiency (WUE) of Peperomia scandens, a CAM plant, was higher than that of the C3 congener P. obtusifolia. This difference has been attributed to differences in stomatal activity between C3 and CAM plants, coupled with differences in the evaporative demand of the atmosphere during which the stomata are open. This explanation has apparently not, however, been experimentally tested. Thus, WUEs were compared in these species in two experiments in which the atmospheric evaporative demand was identical (or nearly so) during the period of stomatal opening (i.e. during the night for the CAM plant and during the day for the C3 species). In both experiments, the WUE of the CAM species was higher than that of the C3 species. These results suggest that factors other than differences in atmospheric environmental conditions must also be responsible for the observed differences in WUE. Because CO2 uptake rates of the CAM species were substantially lower than those of the C3 species, the lower WUE in the CAM species resulted primarily from lower transpiration rates. Lower rates of water loss in P. scandens, relative to rates in P. obtusifolia, were ascribed, in part, to lower stomatal densities. Thus, leaf morphological differences, in addition to differences in atmospheric evaporative demand, help to explain the high WUE typically measured in CAM plants.