The allelopathic potential of rice (Oryza sativa L.) against Echinochloa crus-galli (L.) Beauv. was investigated under both laboratory and greenhouse conditions. A population of 150 recombinant inbred lines (RILs) was derived through single-seed descent from a cross between the indica cultivar AC1423, known to have a strong allelopathic potential, and the aus cultivar Aus196, with low allelopathic potential. The RILs together with the parents were first phenotyped using the laboratory assay known as the relay seeding technique. The screening showed that with regards to allelopathic potential the population phenotype was normally distributed. Two quantitative trait loci (QTLs) were located on chromosomes 4 and 7, explaining 20% of the phenotypic variation. A second relay seeding experiment was set up, this time including charcoal in the perlite. This screening showed that the allelopathic rice varieties did not have any effect on the weed species when grown with charcoal, the charcoal reversing the effect of any potential allelochemicals exuded from the rice roots. The second phenotypic experiment was conducted under greenhouse conditions in pots. Thirteen QTLs were detected for four different allelopathic measurements, located on seven chromosomes, and individually explaining between 5 and 10% of the phenotypic variation. Some QTLs from both experiments located to the same genomic regions. The results show that breeding to increase rice allelopathic potential is possible.