Cotton aphid, Aphis gossypii Glover, is known as a quarantine pest that is hard to control with short periods of fumigation with phosphine (PH₃) or low concentrations of ethyl formate. Moreover, low-temperature fumigation with ethyl formate can lead to phototoxic damage of some perishable commodities. Therefore, a laboratory study was conducted to evaluate the synergistic effect of mixing ethyl formate and PH₃ for the treatment of adults and nymphs of A. gossypii. Combined toxicity was observed and compared with a single dose of either ethyl formate or PH₃. When insects were exposed to 0.5 g/m³ of PH₃ combined with different levels of ethyl formate from 1.6 to 16.3 g/m³ at 5 and 20°C for 2 h, L(Ct)₅₀ and L(Ct)₉₉ values were greatly reduced in comparison with a single dose of either ethyl formate or PH₃. The synergistic ratio (SR) is described as L(Ct) of ethyl formate alone/L(Ct) of ethyl formate + PH₃. The SR values of L(Ct)₅₀ and L(Ct)₉₉ for adult A. gossypii at 5°C were 4.55 and 2.33, respectively. However, at 20°C the SR levels of L(Ct)₅₀ and L(Ct)₉₉ were 2.22 and 1.45, respectively, but still showed significant synergism (significant difference, P< 0.5). This new technology could meet quarantine and preshipment requirements for shorter exposure times and less damage of perishable commodities, and could also be extended for controlling other quarantine pests and thereby be a useful alternative to methyl bromide for fruit and vegetable applications.