Summary.The residues remaining in the soil from repeated annual application of simazine at 2–8, 5–6, and 22–4 kg/ha to uncropped plots on a loam soil were measured by chemical or bioassay methods at various intervals after treatment.The total simazine residue present 12 months after the last of three treatments with 2–8 kg/ha and 8 months after the last of five treatments with 5–6 kg/ha was less than 10% of the annual dose. This rapid decomposition is considered consistent with the soil and climatic conditions. In contrast a much larger residue (a mean value of 1·7 kg/ha) was found on plots sampled 21/2 years after the last of two annual applications of simazine at 22·4 kg/ha and the reduction in the amount of residue during the next 12 months was only of the order of 25%.In all treatments the highest concentration of simazine was found in the surface layers of the soil but measurable residues were detected to 60 cm depth, 31/2 years after the last 22·4 kg/ha application.There was considerable variation in the total residues recovered between replicate plots and between different positions on the same plots in all treatments regardless of the depth of the sample. The possible causes of this variation are discussed.Persistance et pénétration de fortes doses de simazine dans un