Summary.Survival of wild oat seeds (mainlyAvena fatua) under leys on clay soils was investigated in two field experiments. During the first year under ley the number of seeds decreased by 41 % and 86% respectively. Subsequent decreases were less, and after 5 years under ley there were still enough of the original seeds left to produce infestation of a cereal crop. These results suggest that a long ley is not an efficient way of eliminating wild oat seeds from soil, and that a 1‐year ley may be almost as effective as one of 5 years.When a ley was ploughed, the number of wild oats germinating decreased as the number of years under ley and the age of the seeds increased, but was scarcely affected by the weather. In contrast, charlock (Sinapis arvensis) germination did not decrease with increasing age of seeds up to 5 years, but was greatest in years with most rain during April and May.Survivance de semences de Folle Avoine (Avena fatuaL. etA. ludovieiana Dur.) et de Moutarde des champs (Sinapis arvensis L.) dans le sol sous prairie temporai