Summary:Methazole [2‐(3,4‐dichlorophenyl)‐4‐methyl‐1,2,4‐oxadiazolidine‐3,5‐dione] was applied post‐emergence at 2‐1 kg/ha to crops of onion (Allium cepa L.) grown on a sandy loam and the activity present in the surface 0–5 cm of soil assessed by bioassay. In spring‐sown crops, activity at harvest varied from 25 to 78% of the initial acitivity in different years; losses were restricted by low temperature and low rainfall. In autumn‐sown crops, little loss occurred during winter and most of the activity remained in the surface 0‐5 cm. The results emphasize the need for caution in choice an