A marked gain in nitrogen was obtained when plants of Ammophila arenaria L. were grown in pots in dune sand and supplied with nitrogen-free nutrient solution. Nitrogenase activity was associated with the roots but the estimated rates of nitrogen-fixation based upon the acetylene reduction technique were less than one-tenth of those obtained by the Kjeldahl method. Significant rates of acetylene reduction were also demonstrated both for detached roots taken from plants growing naturally in sand dunes and from measurements made in situ in the field, but the estimated rates of nitrogen fixation were much lower than calculated rates of nitrogen gain by actively-growing plants. Some possible reasons for this discrepancy are discussed. Acetylene reduction was greatly stimulated by the addition of glucose, suggesting that nitrogen fixation by micro-organisms in the rhizosphere is limited by the supply of root exudates. Seedlings of Ammophila grown under aseptic conditions and inoculated with Azotobacter showed greatly increased growth compared with that of uninoculated seedlings.