How are high rates of production by Thalassia testudinum Konig maintained in notably nutrient-poor tropical waters? Yield-nutrient supply correlations indicate that a singnificant proportion of the phosphorus, and virtually all nitrogen for leaf growth are taken-up from the sediments, and that growth is generally limited by availability of nitrogen. Considerations of supply and demand suggest that the sediments could not be a primary source of phosphate, but the sediments may act as a “storage bank” for phosphate taken up from the sea water by T. testudinum. It is believed that inorganic nitrogen in the root layer is derived from fixation of gaseous nitrogen by anaerobic bacteria. Maintenance of anaerobic conditions in the root layer appears to be essential for good growth of T. testudinum, and it is suggested that such conditions facilitate nitrogen fixation.