Oxygen concentration in basal stems of Phragmites australis (Cav.) Trin. ex Steudel was measured in situ, in monospecific stands. The mean O2 concentration in basal stems, and mean shoot length above water, decreased in deeper water. Oxygen concentration in basal stems was correlated to shoot length above water at a given water depth, and the mean O2 concentration in basal stems of shoots with similar length above water decreased in deeper water. The results suggest that O2 transport from shoots to below-ground parts is restricted in deep water by: (1) small shoot length above water; (2) long O2 transport distance. No influence of substrate redox potential on O2concentration in basal stems was found. © Copyright 1988 Elsevier B.V.