The concentration of momilactone B in mono-incubated rice seedlings was 2.7 nmol/seedling and that in mixed-incubated rice seedlings was 18.6 nmol/seedling. Thus, the concentration was 6.9-fold greater in mixed-incubated rice seedlings than in monoincubated rice seedlings (Fig. 1B). This result suggests that the production of momilactone B in rice seedlings may be increased by the presence of barnyard grass seedlings. Momilactone B inhibited the growth of roots and shoots of barnyard grass at concentrations greater than 1 mol/L. The effectiveness of momilactone B on the growth inhibition of rice seedlings themselves was less than 1% of that on the growth inhibition of barnyard grass (Kato-Noguchi et al., 2008). As described in “Introduction,” it was reported that momilactone B among rice allelochemicals may play a critical role in rice allelopathy (Kato-Noguchi et al., 2002, 2010; Kato-Noguchi, 2004). Therefore, the increased concentration of momilactone B in rice seedlings under mixed-incubation with barnyard grass (Fig. 1B) may be associated with increased allelopathic activity of rice seedlings (Fig. 1A).