The internal acidity levels of four common North American isoetids, or rosette-form aquatic macrophytes, were monitored under field conditions to determine the presence of crassulacean acid metabolism (CAM). The leaves of Littorella uniflora var. americana (Fern.) Gl. and Isoetes macrospora Durieu were found to have a diurnal fluctuation in titratable acidity characteristic of CAM plants, 124 and 142 μeq g-1 fresh wt., respectively. No variations were detected in the acidity of stems and leaves of Myriophyllum tenellum Bigel. and Lobelia dortmanna L. Litorella and Isoetes were then grown in the laboratory where the diurnal acid rhythm was shown to be due to the accumulation and disappearance of malic acid. Based on the magnitude of the diurnal acid rhythm and existing information on the productivity of these plants, it appears that the carbon assimilated via crassulacean acid metabolism may contribute substantially to their net annual productivity. This appears to be a case for which CAM has been selected directly as a response to carbon stress.