Many non-protein aromatic amino acids have been found in plants. Among the amino acids, L-DOPA (L-3, 4-dihydroxyphenylalanine) and m-tyrosine (L-3-hydroxyphenylalanine) show unique phytotoxic activity. In this review, the action mechanisms of L-DOPA and m-tyrosine as allelochemicals are discussed. The phytotoxicity of L-DOPA is considered to be due to oxidative damage caused by reactive oxygen species generated from the melanin synthesis pathway. Selectivity among species might be achieved by the differential activity of an enzyme, polyphenol oxidase (PPO), catalyzing the conversion of L-DOPA to DOPA quinone in melanin synthesis pathway. Plant species with high PPO activity introduce more L-DOPA in melanogenesis and generate a greater amount of toxic reactive oxygen species. An antioxidant, ascorbic acid, reduced melanin formation by suppressing PPO activity and recovered from L-DOPA toxicity. We found that m-tyrosine had strong phytotoxic activity in lettuce. Interestingly, m-tyrosine also suppressed the growth of barnyardgrass, which is tolerant to L-DOPA. Like L-DOPA, m-tyrosine induced lipid peroxide formation in lettuce; however, this oxidative damage was not rescued by ascorbic acid but by phenylalanine. It is known that certain structural analogues of protein amino acids can be misincorporated into proteins; however, our recent results suggested that m-tyrosine is not incorporated into the proteins of rice root, which is sensitive to m-tyrosine. The mechanism of the phytotoxicity of m-tyrosine is still under investigation.