Summary:The decline of a population ofA. fatuaestablished in September 1971, and not allowed to seed thereafter, was monitored in three successive barley crops Numbers of viable seeds in the son in June fell from 159 m2in 1972 to 1 m2in 1974 declining by 83° in the first, and by 96° in the second year. Seedling numbers fell from 138 m2in 1972 to 9 m2in 1974, declining by 32° in their first and by 89° in the second year. The slower decline of seedling numbers in the first year was attributed to loss of dormancy of seed reserves giving proportionally more seedlings in the second spring.Cultivations had no major influence on the pattern of seedling emergence in the spring, but they did affect population level. Where the stubble was cultivated immediately after seeding in September 1971, twice as many seedlings and three times as many seeds in the soil were present in 1972 compared with delayed autumn cultivations in 1971. From this greater reserve of seeds in the soil more seedlings arose in 1973 and in 1974. Time cultivation in the winter of 1971 resulted in slightly more seedlings in 1972 than did ploughing; this greater population declined more rapidly with line cultivation in 1973 and 1974. It is suggested that with no herbicidal control annual line cultivation will lead to a more rapid build up than ploughing: whereA. fatuais controlled, the decline should be more rapid with tne cultivation. Persistence ofA. fatuaas a weed in arable situations seems related more to survivors shedding seeds than to the persistence of seeds on the soil. Factors which may influence the persistence of seeds in the soil are discus