Seasonal abundance of insects and mites was measured with the aid of insect traps in 4 wooden farm granaries in Manitoba at weekly intervals during May–November for 3 years. Each granary contained 13.6 metric tons of wheat, which was dry and biologically unheated in 2 granaries, and partially damp and biologically heated in the other 2. More insects and mites were found in heated than in unheated granaries. Of the 18 kinds of arthropods trapped, 2 kinds of insects,Cryptolestes ferrugineus(Stephens) and Psocoptera, and 2 kinds of mites,Acarus siroL. andTarsonemus granariusLindquist, occurred monthly in 2 heated granaries. No single species of the insects and mites occurred in every month in unheated granaries;C. ferrugineus, Psocoptera andAndrolaelaps casalis(Berlese) occurred most often.Ahasverus advenaWaltl occurred only in heated granaries during May–November.Lathridius minutus(L.) occurred in unheated granaries during May–July and in heated granaries during May–October.Cryptophagus varusWoodroffe and Coombs occurred in unheated granaries during September–October and in heated granaries during July–October. The continued activity of adults ofC. ferrugineusfrom May–November in both unheated and heated granaries in most years shows unusual adaptation of this important storage pest of cereals which enables it to withstand and adjust to the harsh climate of the Canadian Prairies. Temperature was found to be the most important factor affecting seasonal changes in the populations of most species of insects and mites studied.