Summary.Paraquat dichloride was applied to maize, tomato and broad bean plants as droplets of an aqueous solution and analysed al intervals. It was shown to be degraded under some conditions, but the amount of degradation was least under conditions which would have been most favourable for metabolism of the chemical. The analytical results and results from cation‐exchange paper chromatography and thin layer chromatography showed that the degradation was caused by photochemical decomposition on the surface of the leaves of the plants, not by metabolism. The degradation was not observed in the dark, and only occurred to a significant extent in daylight in the summer.Two photochemical degradation products from methyl‐labelled paraquat‐14C dichloride, viz., 4‐carboxy‐l‐(methyl‐14C)‐pyridinium chloride and methylamine‐14C hydrochloride, were isolated from plants sprayed with the herbicide.Le devenir du paraquat dans le