Abstract The use of a modified atmosphere for fumigation of circular, corrugated steel farm grain storage bins was investigated. The bins were 4.6 m diameter and filled to a depth of 1.5 m with wheat. Details of the concentration histories during purge with the modified atmosphere are given, and data on the effect of gas flow rate during maintenance is also provided. Comparisons were made between bins with and without drying floors, as well as between bins which were unsealed, those which were well-sealed, and those which were sealed by using a plastic film over the grain surface. It was found that satisfactory concentrations could be achieved using purge rates which produce superficial linear velocities above 30 cm/hr. At these rates, the gas volume required to purge the air from the grain is equivalent to the space occupied by the grain bulk (and its interstitial space). After purging, the concentrations may be successfully maintained by a gas rate producing a velocity of 3 cm/hr if a surface covering is used. A surface covering reduced gas maintenance requirements by a factor of 10 compared to that for a ventilated overhead space and by a factor of 2 compared to completely sealing the overhead space.