The interactive effects of three levels of NH 4 –N (50, 500 and 1000 mol l −1) and two levels of phosphate (15 and 50 mol l −1) on growth, nutrient allocation and ammonium uptake kinetics by Phragmites australis (Cav.) Trin. ex Steudel were studied in hydroponic culture in the laboratory. Nitrogen level in the root solution significantly affected the relative growth rate of the plants, the rate being lower at low N (0.026 per day) than at intermediate (0.035 per day) and high N (0.037 per day), but phosphorus did not significantly affect growth. The N : P ratio in the root solution significantly affected the growth rate which was highest at N : P ratios between 10 and 33 on a molar basis. Nitrogen and phosphorus concentration in the plant tissues generally increased with N level in the root solution, but P level had no effect. Plant tissue N : P ratios (on a molar basis) varied between 13.5 in the stems to 28.0 in the leaves and were unaffected by the treatments. Am-monium uptake kinetics were unaffected by N treatment, but V max was significantly affected by P treatment averaging (mean ± 95% confidence limits (CL)) 151 ± 44 mol g −1 root dry weight h −1 in the low-P treatment and 229 ± 70 mol g −1 root dry weight h −1 in the high-P treatment. The overall mean (±95% CL) NH 4 –N uptake kinetic parameters were: V max = 190 ± 20 mol g −1 root dry weight h −1 ; K 1/2 = 21.8 ± 1.8 mol l −1 , and C min = 1.2 ± 0.2 mol l −1 . Mean (±SD) root res-piration rate was 72 ± 22 mol CO 2 g −1 dry weight h −1 and was unaffected by the treatments. The results of the study support the general hypothesis that P. australis is well-adapted for growth in nutrient-rich habitats. However, P. australis is able to acclimate to low nutrient availability by increasing the affinity for ammonium uptake. ©1999 Elsevier Science B.V. All rights reserved.