The aquatic Crassulacean Acid Metabolism (CAM) plant,Littorella unifloravar.americana(Fern.) G1., was the subject of some preliminary investigations to determine (1) if the source of the nocturnally accumulated carbon is endogenous, from respiration, or is exogenous, (2) if this carbon was stored primarily as malic acid, as is typical for the CAM pathway and (3) if the nocturnally accumulated carbon was photosynthetically incorporated during the subsequent light period.Littorellaplants were collected from the field and held in the labor-atory under different levels of CO2, light, and temperature. The diurnal malic acid rhythm in leaves ranged from 46 to 60 μmoles · g−1fresh wt., Radiocarbon studies showed that endogenous CO2(from dark respiration) accounted for 30 to 99% of the nocturnal carbon accumulation. Paper chromatography revealed that the nocturnally incorporated, labelled carbon was primarily in the malate pool, with some labeling of the citrate pool. All of the nocturnally acquired carbon remained within the leaves, as carbohydrate rather than as malate during the light period. These results confirm that nocturnally accumulated carbon is stored as malic acid and is quantitatively incorporated into the plant during the subsequent light period. Evidence indicates that inLittorella, CAM can result in the effective recapture of nocturnally respired CO2and that this endogenous carbon may account for 50% of the nocturnal carbon accumulation under field conditions; however, a more robust experimental approach is needed for confirmation.