To analyze the genetic control of allelopathy in rice, we examined the quantitative trait loci (QTL) controlling allelopathic effects using a F₂ population of the cross between PI312777 and Rexmont, which exert highly suppressive and less suppressive effects respectively, on mono- and di-cot weed species in the field. The allelopathic effects were assessed using water-soluble leaf extracts from rice plants at the six-leaf growth stage. A highly positive correlation was detected between the suppressive effects of rice leaf extracts on the root growth of lettuce and ducksalad. Seven QTL controlling allelopathic effects of rice on lettuce used as a test plant were identified on chromosomes 1, 3, 5, 6, 7, 11 and 12. One of the QTL on chromosome 6 exerted the largest effects on the expression of the allelopathic effects and explained 16.1% of the total phenotypic variation. The other 6 QTL explained the variation in the range from 9.4 to 15.1%. A multiple QTL model estimated that 5 QTL with LOD scores higher than 3.0 explained 36.6% of the total phenotypic variation. Digenic interactions in 5 pairs among 7 QTL were detected. In this paper, we examined the use of allelopathy in rice as a potential method for biological control of weeds.