The chiral requirement on the alpha-methylbenzyl moiety of 2,4-diamino-6-chloro-s-triazine for sufficient inhibition of root growth was similar towards both rice and barnyard millet. With the monoalkylamino series, the most suitable configuration was markedly changed by the substituent on the other amino moiety. However, for the dialkylamino series, the (S)-enantiomer was an active inhibitor. Clear species selectivity between rice and barnyard millet was observed in the series for the (R)-enantiomers, providing high herbicidal activity toward Echinochloa plants and safety toward rice. The enantioselectivity against barnyard millet increased with increasing inhibitory activity of the active enantiomers, following Pfeiffer's rule. R-EtNH (3) controlled the growth of barnyardgrass with leaf-burning (LB) under paddy conditions, and S-EtNH (4) and S-Et2N (20) controlled the growth without LB. The RS-EtNH derivative is an interesting inhibitor controlling the growth of barnyardgrass from the just-germinated stage (by the (R)-enantiomer) to early-middle growth stage (by the (S)-enantiomer).