Changes in the weed flora of Boddington Barn field were monitored during 20 yr of arable cropping after being plowed out of grass in 1960. Seedlings were identified and counted in spring in a regular grid of 151 quadrats (0.1 m2) in 14 of the 20 yr. The number of species recorded ranged from 23 to fifty-three. The maximum number occurred after 4 yr of cultivation. Grassland weeds did not persist long under continuous arable cropping, though Trifolium repens survived the whole 20 years. Ranunculus bulbosus survived 14 yr, Rumex obtusifolius 12 yr, Plantago lanceolata 8 yr and, of the 13 other species recorded, 9 had became extinct by 5 yr. When the permanent pasture was plowed in 1960 2 overlapping areas of the field yielded contrasting arable weeds. One area, probably cultivated sometime between 1940 and 1945, was characterized by Fumaria officinalis, Aethusa cynapium and 4 other weeds: seeds of these species evidently survived 15-20 yr in the soil. The other area which had been cultivated sometime between 1949 and 1960 was characterized by Papaver rhoeas and 3 other weeds. Mean seedling density increased over the period from 29 m-2 in the 1st arable crop to 196 m-2 after 20 years. The season of planting the crop was one of the most important factors determining the weed flora composition in any year. Spring-germinating weeds predominated in spring-sown crops and autumn-germinating weeds in autumn-sown crops. The introduction of weeds in contaminated crop seed was another important factor though other species invaded temporarily by windborne seed.