Isoetes sinensis Palmer (Isoetaceae) is an aquatic or amphibious plant that is critically endangered in China. Previous studies have revealed the crassulacean acid metabolism (CAM)-like photosynthetic pathway occurs com-monly in submerged leaves in genus Isoetes. Water chemistry parameters and the titratable acidity content of the plant extract were measured from samples obtained in the early morning (7:00) and late afternoon (15:00) from two I.sinensis populations in China. One population occurs in the eulittoral zone of a freshwater tidal river at low elevation (134 m) and another occurs in a densely vegetated, high elevation (1 100 m) alpine shallow pool. Significant differences in pH and titratable acidity of the plant extract were detected between the morning and afternoon samples. These changes are associated with diurnalchanges in water chemistry. Our results provide the first evidence for the exist-ence of the CAM pathwa in the East Asian endemic Isoetes sinensis Palmer.The magnitude of fluctuations in the titratable acidity of the plant extract mayb e correlated with the severe carbon limitation imposed on the plants by its aquatic habitat.