Thirty 4-(substituted phenoxy) nitrobenzenes were synthesized and examined for phytotoxicity against barnyard-grass in the light and dark. From the structure-activity relationships and light-requisition for the activity, they were classified into three groups. The diphenyl ethers of the first group, which required light for the activity, were characterized by ortho and para substitutions. The second group compounds having meta or ortho and meta substitutions were active even in the dark. The third group involved compounds substituted at both meta and para positions and their herbicidal activity was inhibited by light. p-Amino analog of nitrofen showed high herbicidal activity.