Uptake of dissolved inorganic carbon (DIC) by roots and rhizomes of Phragmites australis (Cav.) Trin. ex Steudel and the subsequent internal transport and photosynthetic fixation were studied by adding C-14 labeled DIC to the rhizosphere interstitial water and then monitoring the C-14 activity in the interstitial water, the lacunal air and the different plant tissues over the following 70 h. Interstitial water (CO2)-C-14 diffused into the lacunae of the root system and upward within the hollow stems of the plant. The amount of C-14 in all parts of the plant increased with time. The concentration was highest in the basal parts of the plant and decreased with distance from the substrate. Culm nodes and internodes generally contained more C-14 than leaf sheaths and leaf laminae. The data suggest that sediment-derived carbon is predominantly transported via the lacunae to the aerial tissues as gaseous CO2 where it is fixed photosynthetically by green culms and leaf sheaths. Sediment-derived carbon constituted less than 1% of the amount of carbon fixed from the atmosphere by the aerial plant tissues. Apparently, the loss of root respiratory CO2 to the rhizosphere interstitial water quantitatively exceeded the root CO2 uptake. It is suggested that the utilization of sediment-derived carbon may be of significance for young sprouting shoots.