The term seed bank is used herein to refer to the store of seeds buried in the soil (Harper 1977) both in the general sense referring to all species and in a restricted sense referring only to the propagules of a species. The seed bank is a dynamic entity that varies in composition, size, and distribution. By examining the contributions of one species to the seed bank, one can better understand the variation encountered in seed banks in general. Common ragweed (Ambrosia artemisiifolia L.) is an important weed throughout much of temperate North America, often cited as a dominant in early stages of succession (Oosting 1942, Keever 1950, Bard 1952, Quarterman 1957, Bazzaz 1970). Common ragweed has been noted for the production of pollen and was listed as the most common cause of hay fever in North America (Basset and Frankton 1971, Wodehouse 1971). Reviews of life history and importance of common ragweed were given by Basset and Crompton (1975) and Gebben (1961). The taxonomy of the genus Ambrosia was described by Payne (1962). Common ragweed is an annual that overwinters by seeds, and thus is expected to have a large seed bank. Furthermore it is well suited for studies of seed banks, because it possesses large readily identifiable seeds. Knowledge of germination requirements and potential longevity of common ragweed seeds can be found in the literature (Baskin and Baskin 1985). The objectives of this study were: (1) to examine how the size of common ragweed's seed bank fluctuated throughout a year; (2) to describe how the seed bank varied with respect to the vertical soil profile: and (3) to derive inferences about average longevity and the fate of seeds.