Studies concerning Furcraea species behaviour and physiology are scarce, even though these plants occupied 1.11% of the total surface area cultivated by permanent crops in Colombia in 2007. Both leaf sap pH values and titratable acidity for two species of Furcraea plants were measured hourly throughout a whole day in a field experiment in Tunja, Colombia. Furcraea castilla and F. macrphylla were the species being studied; they are frequently cultivated in the area to obtain plant fibres. The leaves were macerated to extract the leaf sap which was titrated with 0.05 N NaOH. There was a clear diurnal pH and titratable Articulo Original /Original Article Orinoquia 13(1):5-13, 2009 Revista ORINOQUIA Universidad de los Llanos Villavicencio, Meta. Colombia Volumen 13 No 1 Ano 2009 6 organic acid content circadian rhythm in the leaves. The results revealed diurnal changes in titratable acidity levelsand pH values exhibiting Crassulacean acid metabolism (CAM), exhibiting pH values above the mean between 11:00 and 20:00 hours. The pH was below the mean during the remaining time. Titratable acidity presented an opposite pattern during the same time intervals.