Technical Abstract: Fifty-seven rice entries including current and former commercial rice cultivars, semi-dwarf mutants of commercial rice cultivars, advanced breeding lines, foreign lines and cultivars suspected or proven to have weed-suppressive characteristics, and crosses or hybrids between commercial cultivars and weed-suppressive lines were evaluated in the field for natural barnyardgrass (BYG) suppression at Stuttgart, AR. As a group, the foreign PI lines generally suppressed BYG better and maintained higher yields in the presence of BYG than did the other rice groups. Semi-dwarf mutants generally suppressed BYG less than did the other rice groups. While many commercial cultivars and advanced breeding lines suppressed BYG very little, a few of them were statistically as suppressive to BYG as were the most suppressive foreign lines indicating that existing commercial rice germplasm may be a valuable genetic source for natural weed suppression activity and should be evaluated routinely as part of a complete crop improvement program for rice.