Concentrations of momilactone B in rice seedlings increased significantly at nutrient concentrations lower than 0.1 N (Fig. 3). The momilactone B concentration in the rice seedlings incubated in 0.01 N medium was 2.6-fold greater than that of rice seedlings incubated in 1 N (normal strength) medium, which suggests that the growth conditions under low nutrient concentration may increase the momilactone B production in the rice seedlings. The elevated rice allelopathic activity by low nutrient conditions (Fig. 2) may be caused by the accumulation of momilatone B in rice seedlings (Fig. 3) because of the strong allelopathic activity of momilactone B against barnyard grass (Kato-Noguchi et al., 2008, 2010).