UV-irradiation increased the concentration of momilactone B in shoots and roots of rice seedlings, and increasing the irradiation increased the concentration. The concentration in 90-min UV-irradiated shoots and roots, respectively, was 31.8- and 3.6-fold higher than that in non-irradiated shoots and roots. After UV-irradiation the concentration of momilactone B in rice shoots decreased. There was, however, an accumulation of momilactone B in the medium in which UV-irradiated seedlings were grown. Five days after UV-irradiation, momilactone B in the medium was at a level 2.5 times greater than on day 0, which was 47% of momilactone B in the seedlings, suggesting that rice may actively secrete momilactone B into medium. Therefore, UV-irradiation increased not only production of momilactone B in rice seedlings but also secretion of momilactone B into rice rhizosphere. As momilactone B acts as an antimicrobial and allelopathic agent, secretion of momilactone B into the rhizosphere may provide a competitive advantage for root establishment through local suppression of soil microorganism and inhibition of the growth of competing plant species.