The methanolic crude leaf extract of Acacia melanoxylon shows inhibitory effects on seed germination, shoot growth, root growth and biomass of black gram (Phaseolus mungo L. Co 1) and weeds (Cassia occidentalis L. and Cyperus rotundus L.). The post emergence treatment was given on the 8th day after sowing and 4-6 days after seedling emergence. After postemergence treatment with 20% extract concentration, wilting symptoms appeared in P. mungo and C. occidentalis while necrosis occurred in C. rotundus. The seedling growth was inhibitory at all concentrations (5%, 10%, 20%) as compared to control plants. This inhibitory effect may be due to the presence of allelochemicals like phenolics in the A. melanoxylon leaves. Different types of phenolic acids were identified.