The oxygen content of the intergranular spaces in bulk grain has a considerable effect on the state of the microporganisms since the great majority of these are aerobes (except yeasts, a few moulds and bacterial varieties). First invitro experiments with controlled atmosphere, in sealed containers, pointed out that progressive anoxy is most operative condition whereas carbon dioxide in various contents can reduce or enhance the suppressive effect of oxygen deficiency. Second, assays were carried out with soybean stored in various controlled atmospheres (air-tight condition, partial vacuum or supplies carbon dioxide...) as suitable methods for an oxygen-free condition. Specific and selective action of these modified atmopheres resulted in original and restricted series among the spontaneous microflora; dynamics of these are obviously dependent on other ecological factors. According to such a complementary pattern, a practical method of grain storage is suggested as the easiest and cheapest on the farm scale: damp corn or rehumidified wheat grains are crushed before storing into compressed batches.