A ten ton aluminum silo filled with maize has been fumigated with four tablets of phostoxin per ton. The surface was covered with a polythene sheet after applying the tablets to make the system airtight. Phosphine concentrations were followed at nine sampling points at different periods after fumigation. Effectiveness was tested by using the sequential sampling technique before and four days after fumigation. Virtually no phosphine gas reached the outlet. The concentration reached a maximum in the fifth hour after fumigation and in some points in the twenty-first hour. Very low concentrations were achieved at all sample points after 53 hours. The fumigation was very effective against the adult and pre-adult stages of the insect pests in the maize except at the outlet.