The relationship between water-use efficiency, measured as the transpiration ratio (g H₂O transpired g^–1 above- plus below-ground dry mass accumulated), and ¹³C / ¹²C ratio (expressed as δ¹³C value) of bulk biomass carbon was compared in 15 plant species growing under tropical conditions at two field sites in the Republic of Panama. The species included five constitutive crassulacean acid metabolism (CAM) species [Aloe vera (L.) Webb & Berth., Ananas comosus (L.) Merr., Euphorbia tirucalli L., Kalanchoë daigremontiana Hamet et Perr., Kalanchoë pinnata (Lam.) Pers.], two species of tropical C₃ trees (Tectona grandis Linn. f. and Swietenia macrophylla King), one C₄ species (Zea mays L.), and seven arborescent species of the neotropical genus Clusia, of which two exhibited pronounced CAM. The transpiration ratios of the C₃ and CAM species, which ranged between 496 g H₂O g^–1 dry mass in the C₃–CAM species Clusia pratensis Seeman to 54 g H₂O g^–1 dry mass in the constitutive CAM species Aloe vera, correlated strongly with δ¹³C values and nocturnal CO₂ gain suggesting that δ¹³C value can be used to estimate both water-use efficiency and the proportion of CO₂ gained by CAM species during the light and the dark integrated over the lifetime of the tissues.