The phytotoxic monoterpenes, camphor, pulegone, and borneol, exhibit a marked synergistic action when used in combination. On a molar basis, these phytotoxic concentrations are enhanced from 40 to 100 times by using two compounds simultaneously. The toxicity of these compounds is about the same for radish (Raphanus sativus L.) and wheat (Triticum vulgare Vill.) but is less by an average of tenfold for oats (Avena sativa L.). When the compounds are used singly, they are from 3 to 12 times as effective in inhibiting root elongation as they are in inhibition of germination.