Nitrogen flux from sediment of a shallow lake and subsequent utilization by water hyacinth (Eichhornia crassipes [Mart] Solms) present in the water column were evaluated using an outdoor microcosm sediment-water column. Sediment N was enriched with 15 N to quantitatively determine the movement of NH 4 -N from the sediment to the overlying water column. During the first 30 days, 48%o of the total N uptake by water hyacinth was derived from sediment 15NH 4 -N. This had decreased to 14% after 183 days. Mass balance of N indicates that about 25% sediment NH 4 -N was released into the overlying water, but only 17% was assimilated by water hyacinth. NH 4 -N levels in the water column were very low, with very little or no concentration gradients. NH 4 -N levels in the interstitial water of the sediment were in the range of 30-35 mg L -for the lower depths (>35 cm), while in the surface 5 cm of depth NH 4 -N levels decreased to 3.2 mg L -1 . Simulated results also showed similar trends for the interstitial NH 4 -N concentration of the sediment. The overall estimated NH 4 -N flux from the sediment to the overlying water was 4.8 Ag cm-2 day-', and the soluble organic N flux was 5.8 /Ag N cm -2 day-1 . Total N flux was 10.6 /Ag N cm-2 day-1 . 'Florida Agricultural Experimental Stations Journal Series No. 8205.