Technical Abstract: This study investigated the effect of atmosphere modification, being a widely adopted means of insect control in stored products, on the efficacy of Beauveria bassiana against one of the most difficult to control pests, Tribolium castaneum. Oxygen reduction to 5% (± 1%) as opposed to CO2 elevation to 40% (± 2%) for the first 72 hours of fungus exposure resulted in significantly greater larval mortality than fungus exposure under ambient atmospheres. Both treatments reduced pupation of older larvae suggesting that slowed development may be a beneficial factor for fungal efficacy. Neither oxygen reduction nor carbon dioxide elevation significantly affected the mortality of adult beetles. Carbon dioxide elevation significantly reduced B. bassiana¿s germination and growth rates, but oxygen reduction did not.