The four tolerant and one susceptible plant to oxyfluorfen were selected from 26 species and investigated for the inhibition of protox activity, the PPIX accumulation, and the activity of antioxidative enzymes by oxyfluorfen treatment. When treated, the oxyfluorfen-tolerant plant species showed a less decrease in fresh weight and height than the susceptible one. The susceptible barnyardgrass showed more inhibition of protox activity due to the treatment of oxyfluorfen than the tolerant species. Especially at the concentration of $10^{-6}M$, protox activity of the susceptible barnyardgrass was inhibited completely, but tolerant species maintained $25{sim}45%$ of the activity. Under the light and dark condition, the susceptible barnyardgrass showed more PPIX accumulation than the tolerant.