Ozone is a highly oxidizing gas with insecticidal activity and it is a potential alternative to conventional fumigants, such as phosphine and methyl bromide, for managing stored product insects. Susceptibility of the merchant grain beetle, Oryzaephilus mercator (Fauvel; Coleoptera: Silvanidae), an important pest of stored products, to ozone treatments is unknown. In this study, we investigated the efficacy of ozone for controlling O. mercator. We determined concentration-mortality relationships for all stages of O. mercator exposed to 100-400 ppm for 1 h (1 g/m3 = 467 ppm). We also determined time-mortality relationships for adults exposed to 100 ppm for 1-6 h. Mortality was recorded as percentages of eggs that failed to hatch 10 days after treatment (DAT), larvae or pupae that failed to develop into adults 20 or 15 DAT, respectively, and adults that died 2 DAT. Generally, mortality increased with an increase in ozone concentration. Mortality was higher when insects were treated without food. When food was not provided, a minimum of 11030 ppm for 1 h is required to kill 99% of eggs, the most tolerant stage, whereas 500 ppm for 1 h is required to kill 99% of larvae, the least tolerant. When provided with food, adults were the most tolerant and larvae the least tolerant. Adults require exposure time of 7.7 h of 100 ppm ozone to kill 99% of insects in the absence of food. The work reported suggests that ozone could be an alternative fumigant for the management of all O. mercator life stages.