Strong resistance to phosphine (PH₃) in the rusty grain beetle, Cryptolestes ferrugineus (Stephens) (Laemophloeidae: Coleoptera) poses a serious risk to stored-grain biosecurity. Resistant populations hold risk of surviving in PH₃ fumigation, particularly in storage structure that limits achieving very high concentrations of PH₃, demanding the need for alternative fumigation strategies. Cofumigation with PH₃ and carbon dioxide (CO₂) is one alternative approach that has the potential to be used widely. CO₂ fumigation of adults of strongly PH₃-resistant reference strain of C. ferrugineus, for 48 h, showed that the effective concentration (LC₅₀) of CO₂ was 30.99%. This 30% level of CO₂ in combination with PH₃ decreased the LC₅₀ of PH₃ from 6.7 mg/liter to 0.84 mg/liter, an eightfold increase in PH₃ efficacy relative to PH₃ fumigation in normal air. The LC_99.9 decreased from 16.2 mg/liter to 5.8 mg/liter, a 2.8-fold increase in PH₃ efficacy. Comparison of mortality response data of PH₃ alone and the PH₃ + CO₂ mixture confirmed that CO₂ enhances the toxicity of PH₃ synergistically in addition to exerting its own toxicity. These results were validated against three independently field-derived strains of strongly resistant C. ferrugineus that confirmed that observed enhancement in toxicity with the PH₃ + CO₂ mixture was consistent, irrespective of differences in resistance phenotypes and inherent tolerance levels. Results of the current study provide further opportunities to develop new commercially viable strategy to control strongly PH₃-resistant C. ferrugineus.