BIOSIS COPYRIGHT: BIOL ABS. Silos of 10.55 m in diameter and 12 m in height were made of corrugated circular steel for corn or sorghum storage. Some were equipped with circulatory systems consisting of a plastic pipe 10.16 cm (4 in) in diameter from the top to the bottom. Before loading, two sets of gas sampling lines, 4 mm in diameter, containing five lines each, were installed in the center and on the circumference of the silo. The openings in each set were at depths of 0, 3, 6, 9 and 12 m. The gas samples were measured using an EC80 phosphine monitor every 24 h after fumigant application. Phosphine was generated by Detia bag blankets containing 57.6% aluminum phosphate placed on the surface of the grain mass. Generally, the highest concentration of phosphine was obtained 24 h after application in each silo. In silos without a circulatory system, phosphine penetrated down to the bottom through interstitial spaces in the corn or sorghum column. The concentration at the surface, where application was made, was the highest at 24 h. The concentration dropped thereafter due to diffusion, adsorption and decomposition. The deeper into the column, the lower the phosphine concentration was. In each measurement during 24 120 h after fumigant application, the ratios of the highest to lowest phosphine concentration was between 1:0.12 and 1:0.14, i.e., the lowest phosphine concentration was only 2 14% of the highest. In silos with a circulatory system, phosphine was sucked into an aeration pipe from the top and sent to the bottom of the silo, then moved upwards through the grain mass. During 24 120 h after fumigant application, the ratios of the highest to lowest phosphine concentration was within 1:0.81 and 1:0.98, i.e., the lowest concentration was 81 98% of the highest. In the circulatory silo, mortality rates for test insects placed on the surface of grain or insects obtained from grain samples showed 100% control.