The khapra beetle, Trogoderma granarium (Everts), is considered one of the most destructive insect pests of wheat storages worldwide, whereas phosphine fumigation is common to manage this pest. The efficacy of phosphine use largely depends on its concentration, exposure time and target species. However, building an effective relationship between phosphine concentration and exposure time for T. granarium is of great importance. This research was conducted to evaluate the effects of concentration and time on phosphine toxicity against susceptible or resistant strains of T. granarium. Both strains of T. granarium were exposed to phosphine gas at various concentrations and exposure times in a laboratory. Our results showed that the exposure time was the more important variable than the phosphine concentration against both T. granarium strains; in contrast, the concentration was comparatively more important against the resistant strain than the susceptible one. This study highlights the importance of concentration and exposure time in phosphine fumigation efficacy against T. granarium.