The development of acetylene reduction activity (ARA) formed prior to germination on the surfaces of seeds was investigated with various kinds of seeds. Cocklebur (Xanthium pennsylvanicum Wallr.) seeds exhibited ARA regardless of dormancy status when exposed on a river sand stratum in a Petri dish, although the seeds themselves did not have ARA. The ARA on cocklebur seeds increased with time, reaching a maximum after ca 6 days, and the increase in ARA corresponded with that in the number of N2-fixing bacteria living on the seed surface. The population of bacteria on the seed surface differed from that in the sand stratum: N2-fixing bacteria were more dominant on the seed surface than in the sand incubated with or without seeds. Similarly. ARAs were associated with the seeds of barley, rice. grape, melon, eggplant and cherry, exposed to river sand. Cocklebur and eggplant seeds showed high ARAs in anoxia, whereas the others showed high ARAs in hypoxia. The ARA of the cocklebur seedlings grown from the seeds which had been in contact with the river sand was much higher than that of the non-contacted seedlings. It was concluded that a bacterial habitat, the "spermosphere", is established around soaked seeds prior to germination and promotes the subsequent development of the rhizosphere.