AbstractAs part of an investigation into the efficacy of the systemic insecticide Pestox III (octamethylpyrophosphoramide, Schradan) as a. protective agent against aphis‐borne virus rosette in groundnuts, the movement of32P‐labelled Pestox was followed. Plants were sampled at random in groups of ten at intervals up to 25 days after spraying. The parts above ground were treated so that Pestox‐contained radioactivity and other radioactivity could be assayed separately. Roots, nuts and soil samples were tested for total radioactivity only. It was found that decrease in radioactivity in the parts above ground was due to loss of Pestox as a whole. The largest loss was due to evaporation immediately after spraying and a further loss took place by movement into the roots, nuts and soil. There was no decomposition of Pestox until more than 18 days after spraying. The radioactivity in the roots and soil samples increased steadily with