Phytotoxins from lantana leaves were extracted in aqueous media adjusted to pH 4, 7, or 10. All three leaf extracts showed considerable phytotoxic activity in duckweed growth bioassay. The acidic and the neutral extracts of lantana leaves at 10 g fresh weight/L were more phytotoxic to duckweed growth than the alkaline extract at the same concentration. Phenolic compounds present in lantana leaves were isolated by alkaline hydrolysis of the crude leaf extracts followed by separation of the various components in ether or aqueous media. The fraction containing the phenolics showed considerable phytotoxic activity against duckweed growth. In addition, the fraction containing the nonpolar substances separated from the acidic and the neutral crude extracts also showed significant phytotoxic activity against duckweed growth. No attempt was made to isolate and identify the phytotoxic component from the nonpolar fraction. Characterization of the phenolic fraction by HPLC revealed the presence of at least 14 phenolic compounds. All three leaf extracts contained the same array of phenolic compounds. However, the quantity of phenolic compounds extracted varied with the pH of the extraction medium. The acidic extract contained p-hydroxybenzoic acid (0.09 mM) as the most abundant phenolic compound, whereas the neutral and the basic extracts contained p-coumaric acid (0.11 and 0.26 mM, respectively) as the most abundant phenolic compound. All phenolic compounds, except p-hydroxybenzoic acid, proved phytotoxic to duckweed growth at a concentration of 1 mM or lower. Salicylic acid was the most phytotoxic of the phenolic compounds detected in lantana leaf extracts. The phytotoxicity of lantana leaf extracts is probably partly due to a complex interaction of all phenolic compounds present.