Phosphine (PH₃) and ethyl formate (EF) are two potentially powerful postharvest fumigant insecticides.We investigated the effectiveness of both PH₃ and EF as fumigants at all developmental stages of the potato tuber moth Phthorimaea operculella Zeller, and we also studied the synergistic effects of these fumigants under controlled atmospheres of 50 and 80% oxygen (O₂). The larval stage of P. operculella was the most susceptible to fumigation with PH₃ at both 5°C and 20°C. All of the developmental stages showed greater susceptibility to PH₃ at 20°C than at 5°C, whereas the susceptibility of adult P. operculella to this fumigant was not affected by temperature. The toxicity of EF did not differ with temperature for any of the P. operculella developmental stages. The atmospheric oxidation of PH₃ increased the toxicity of this fumigant toward all developmental stages at both temperatures. In contrast, no differences in toxicity were observed for oxidized EF compared with EF alone at any developmental stage. In conclusion, using fumigation tests, we showed that atmospherically oxidized PH₃ was much more effective against P. operculella than PH₃ alone, demonstrating a synergistic effect for this fumigant and O₂. Therefore, treatment with PH₃ and high concentrations of O₂, as described in this study, could be useful for managing the postharvest pest P. operculella.