[Show abstract][Hide abstract] ABSTRACT: An inhibitory effect of the phenolic compounds released from the roots of K21, a newly bred potent allelopathic rice, was investigated in comparison to its male parent, Kouketsumochi, and its female parent, Dongjinbyeo. The inhibitory rates of the root exudates of K21 and both of its parents, collected at 5 day intervals, on the shoot growth of Echinochloa crus-galli var. praticola exhibited an increasing trend from 19–34 days after seeding (DAS). However, the root exudates of K21, at concentrations of 0.5, 1.0, and 2.0%, showed a greater inhibitory effect than those of its female parent on the growth of E. crus-galli var. praticola at the earlier collection dates. Nine phenolic compounds were identified; the total amount of phenolic compounds in the exudates increased from 19–34 DAS in both K21 and its male parent, but not much in the female parent. The subsequent increase in the amount of total phenolic compounds in the root exudates of the later investigation dates and the trend of increasing inhibitory rates on the shoot growth of E. crus-galli var. praticola at higher exudate concentrations imply that the increased amount of phenolic acids in the root exudates of K21 secreted on the subsequent dates could be related to its greater allelopathic effects.