U.S. Hard Red (Grade II) and Western White wheat stocks, stored in Concrete Bins, were fumigated with Hydrogen phosphide (Phostoxin) at the dosage rates of 4, 3, 2 and 1 tablets per tonne. For each dosage rate, five bins were fumigated at random. Wheat stocks in all the bins were heavily infested by Sitophilus oryzae (L.), Rhyzopertha dominica (F.), Trogoderma granaria (Everts), Tribolium castaneum Hbst., Oryzaephilus surinamensis Waterh., Cryptolestes minutus and Psocids. In all cases, an exposure period of eleven days was allowed. For assessment of the insect mortality obtained in the respective experimental bins, grain samples were drawn out by a Thermo-sampler before and after the fumigation. Action of PH3 on the control of hidden insect infestation was estimated by incubation tests. Effects of the fumigation on grain viability were also assessed. (The) various dosages of Phostoxin tried, gave complete mortality of all stages of the insect pests infesting the wheat stocks in the experimental bins. Complete mortality of insects also occured in the untreated bins, suggesting thereby that Ph3 vapour permeated through the inter-bin walls. Phostoxin did not impair viability of the treated wheat lots. There were strong indications that it possed (a) high degree of toxicity to hidden insect infestation also.