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SUMMARY (1) Measurements have been made of seasonal variation in the density and composition of the reservoir of germinable seeds present in surface (0-3 cm) soil samples collected at 6-weekly intervals from ten ecologically-contrasted sites in the Sheffield region. (2) The procedure was not designed to provide a complete assessment of the seed flora, and the methods were found to be ineffective in recovering germinable seeds of those species (e.g. Endymion non-scriptus, Viola riviniana, several Umbelliferae) in which there is only a brief interval between fulfilment of a chilling requirement and the onset of germination. (3) The techniques adopted were particularly suitable for the detection of persistent seed banks (i.e. those in which some of the component seeds are at least 1 year old), and also allowed recognition of species in which there is a transient accumulation of detached germinable seeds during the summer. (4) Comparison of the results obtained for populations of the same species in different types of habitat suggests that seasonal variation in seed number is a function of the species rather than of the environment. (5) It is concluded that the major evolutionary force determining the nature of the seed bank is the selective advantage derived from mechanisms of seed dormancy and germination which allow seedlings to evade the potentially-dominating effects of established plants. (6) From the data collected in this study, four types of seed bank (Types I-IV) have been recognized, and an attempt has been made to assess their ecological significance. (7) The transient seed banks (Types I and II) are adapted to exploit the gaps created by seasonally-predictable damage and mortality in the vegetation, whilst the persistent seed bank (Type IV) confers the potential for regeneration in circumstances where disturbance of the established vegetation is temporally and/or spatially unpredictable. A second type of persistent seed bank (Type III) has characteristics intermediate between those of Types I and IV, and contains some seeds which germinate soon after release and others which are more persistent in the soil. (8) A feature of the results was the lack of a general correspondence between the species-composition of the seed flora and that of the associated vegetation. At certain sites, substantial persistent seed banks were detected for species which were either extremely scarce or did not occur at all in the established vegetation. (9) Both transient and persistent types of seed banks were represented at each of the ten sites; this is consistent with the hypothesis that complementary mechanisms of regeneration are involved in the maintenance of floristic diversity.