Wheat in seven tanks of an oil tanker was fumigated by applying aluminum phosphide round tablets at the rate of 1.06 g/m3. Tablets applied by probing into the wheat gave better gas distribution and a more efficacious fumigation than those applied on the surface. Phosphine gas moved slowly downward through the wheat mass beyond the 9-m depth in the center tanks and remained at low concentrations even after 24 days at the 17-m depth. Only one center tank, where the fumigant was probed to a depth of 4.3 m, contained no live insects. Pressure-vacuum readings recorded on tanks over a 10-h daytime period ranged from +12.5 to -15 mm of Hg. For each 1°C rise in overhead temperature, there was a corresponding rise of 1 mm of Hg in pressure. No phosphine was detected in working areas or living quarters of the tanker. Forced ventilation of the fumigated holds for 1 hour before unloading was sufficient to remove phosphine from their head space above the grain mass; however, 24 h was required for natural ventilation. Phosphine was not detected in the overhead space of tanks while the pneumatic conveying system was in operation. Phosphine residues were detected on all postfumigation samples of wheat. Residues ranged from 0.001 to 0.2 ppm on samples taken from the discharge spout of the pneumatic conveying system. Only two samples out of 35 exceeded the 0.1-ppm tolerance.