Abstract Fenitrothion at 2.5, 5.0 and 6.0 ppm was compared with malathion at 12.0 and 18.0 ppm for the protection of bulk wheat in vertical bin silos, and 10 ppm fenitrothion was compared to 18.0 ppm malathion in horizontal bulk depots. In both types of storage the persistence of the insecticides could be correlated with the temperature and moisture content of the grain. Under the conditions of silo storage, both protectants remained effective against Rhyzopertha dominica, Sitophilus granarius and secondary pests for as long as 19 months. Under the more severe conditions of horizontal bulk storage the higher applications were effective for only 6–7 months. Fenitrothion at one half the application rate of malathion appears to give equivalent protection against the deterioration of stored wheat caused by attack from stored product insect pests.