Laboratory and plant house experiment were conducted to study the allelepathic effact of Acacia ,
an invasive plant species in Brunei Darussalam, on germination and growth of two local paddy varieties. Germination,relative growth rates and biomass allocations of paddy Lalia and Pusu were determined using a series of aqueous leaf extract concentration and aqueous soil extract concentration from A. mangium plantation and heath forest.Lalia appears to be the most sensitive target species as its germination and relative growth rates based on elongations lengths were affacted by all the three type of extract. Mean percentages germination of seed treated with A. mangium leaf and A. mangium and heath forest soil extract significantly decreased in Lalia and Pusu as extract concentration incressed to 10/12%. Mean RERs of seeds treated with 12% of A. mangium leaf extract were significantly slower in both Lalia and Pusu as compared to the control. Both types of soil extract signiificantly decreased RERs in Lalia but not Pusu. At 15% of leaf extract concentration, Laila seedlings allocated a higher proportion of dry mass to roots but a lower proportion of dry mass to shoots than in other treatments but this differential allocation did not translate into greater final total dry biomass of faster growth rate . Acacia mangium negatively affected germination and growth of paddy. It is suggested that careful planning needs to be undertaken before using invasive species in any integrated land use systems.