Crassulacean acid metabolism (CAM) was observed in three species of tropical ferns, the epiphytes Microsorium punctatum and Polypodium crassifolium and the lithophyte Platycerium veitchii . Polypodium crassifolium and P. veitchii exhibited characteristics of weak CAM. Although no net nocturnal CO2 uptake was observed, the presence of CAM was inferred from nocturnal increases in titratable acidity of 4.7 and 4.1 µequiv (g fr wt)–1 respectively, a reduction in the rates of net CO2 evolution during the first half of the dark period, and the presence of a CAM-like decrease in net CO2 uptake during the early light period. In M. punctatum net CO2 uptake during the first half of the dark period was accompanied by an increase in titratable acidity of 39.2 µequiv (g fr wt)–1 and a pronounced reduction in net CO2 uptake during the early light period. When water was withheld from P. crassifolium and M. punctatum , net CO2 uptake during the light was reduced markedly but there was no change in the extent or patterns of CO2 exhange in the dark. As a consequence, the proportion of carbon gained due to CO2 fixation in the dark increased from 2.8 and 10% to 63.5 and 49.3%, respectively (100% being net CO2 uptake during the light plus the estimated CO2 uptake during the dark). After 9 days without added water, dark CO2 uptake was responsible for the maintenance of a net 24 h carbon gain in P. crassifolium . Platycerium veitchii , P. crassifolium and M. punctatum exhibited carbon isotope ratios of between –25.9 and –22.6‰ indicating that carbon isotope ratios may not, by themselves, be sufficient for the identification of weak CAM. We suggest that CAM may be more prevalent in tropical epiphytic and lithophytic ferns than currently envisaged.