AbstractThe phytotoxicity of someO‐alkyl methylphosphonofluoridates has been studied by examining the effects of the compounds and their likely breakdown products on the germination of wheat seed and on the growth of wheat seedlings (14 to 18 days old) in hydroponic culture. The fourO‐alkyl methylphosphonofluoridates (ethyl;isopropyl; 3,3‐dimethylbut‐2‐yl(pinacolyl) andcyclohexyl) were found to be phytotoxic and the phytotoxicity was shown to be a property of the compounds as such and not of their breakdown products. The phytotoxicity of the phosphonofluoridates was apparently influenced by the nature of theO‐alkyl group and tended to increase as follows: ethyl