Experiments aimed at examining the effect of aqueous extracts of three weed species on nodulation and nodule function of soybean cv. Melrose have been carried out at the Laboratory of Plant Physiology and Biotechnology, Department of Agronomy and Soil Science, University of New England, Australia. Aqueous extracts of fresh weed material (Amaranthus powellii, Cyperus rotundus and Paspalum dilatatum) at the concentration of 10% (w/v) were added to a minus-nitrogen Hoagland’s nutrient solution in which the soybean plants were grown with 14 hours day length, day and night temperatures of 28 and 20°C, respectively, light intensity of 790 mol/m2/s, and the relative humidity of 65%. The plants were kept for three weeks prior to the measurement of activity of nitrogenase enzyme and ammonium content of the root nodules. All weed extracts tested resulted in impairment of soybean nodulation and nodule function as indicated by reduced activity of nitrogenase enzyme activity (acetylene reduction assay - ARA). Although amaranth extract was most inhibitory to the nitrogenase enzyme activity, it was less inhibitory than nutgrass extract in reducing the total ammonium content of the soybean root nodules.