Twelve new quarternary ammonium compounds were synthesized and their plant growth retarding activities were examined. Among the candidates, N,N,N-trimethyl-l-methyl-3-(3′,3′5′-trimethylcyclohexyl)- and N,N,N-trimethyl-l-methyl-3-(3′,3′,5′,5′-tetramethylcyclohexyl)-2-propenylammonium iodides were the most effective to suppress the growth of rice and cucumber seedlings, and their activities were far stronger than those of any growth retardants hitherto known.