Four common medicinal and two leguminous plant species from Southeast Asia were selected from a preliminary survey and examined for their potential use in paddy weed control. The screening indicated that all species exhibited strong allelopathic activities, the strongest of which was Nerium oleander. Allelopathic potentials of plant parts were ranked in the order: leaf>root>stem. The only exception was for Alocasia cucullata, where the stem displayed the highest potential. In a bioassay, N. oleander and Helianthus tuberosus showed the highest suppression of germination and growth of Echinochloa crus-galli (barnyardgrass) and Monochoria vaginalis (monochoria). Other than for Passiflora incarnata, the spontaneous growth of paddy weeds was significantly suppressed in a greenhouse at 1 t ha-1. At 1.5 t ha-1, all examined species markedly reduced weed plant growth and the dry weight of weeds by 60–100% and 70–100%, respectively. In paddy fields, weed biomass was reduced by 70–80%, and rice yield was increased by an average of 20% compared with the control. Stylosanthes guianensis, a leguminous plant widely used in Southeast Asia and Africa as a cattle feed, cover crop, mulch, and for soil improvement, gave the greatest increase in rice yield (25.8%). These plants might be useful as natural herbicides and might also contain numerous growth inhibitors that could be used for the development of biological pesticides.