Hemiepiphytic species in the genera Clusia and Ficus were investigated to study their mode of photosynthetic metabolism when growing under natural conditions. Despite growing sympatrically in many areas and having the same growth habit, some Clusia species show Crassulacean acid metabolism (CAM) whereas all species of Ficus investigated are C3. This conclusion is based on diurnal CO2 fixation patterns, diurnal stomatal conductances, diurnal titratable acidity fluctuations, and δ13C isotope ratios. Clusia minor, growing in the savannas adjacent to Barinas, Venezuela, shows all aspects of Crassulacean acid metabolism (CAM) on the basis of nocturnal gas exchange, stomatal conductance, total titratable acidity, and carbon isotope composition when measured during the dry season (February 1986). During the wet season (June 1986), the plants shifted to C3-type gas exchange with all CO2 uptake occurring during the daylight hours. The carbon isotope composition of new growth was-28 to-29‰ typical of C3 plants.