The efficacy of an existing cold disinfestation postharvest treatment targeting Ceratitis capitata (Wiedemann) in citrus was determined for the marula fly, Ceratitis cosyra (Walker). The cold tolerances of C. capitata and C. cosyra were first quantified in artificial diet at 3.5 °C at different exposure periods for up to 18 days. Ceratitis capitata was found to be more cold tolerant than C. cosyra. At 3.5 °C, the duration to achieve 99.9968% mortality was calculated to be 11.57 days for C. capitata and 9.10 days for C. cosyra. Under an existing C. capitata cold treatment schedule at 1 °C for 14 days, the conditions required for complete mortality of the third larval stage of C. cosyra in orange, Citrus sinensis (L.) Osbeck cv. Valencia, were then determined. No survivors of C. cosyra in oranges were recorded beyond 11 days of cold treatment at 1 °C. The efficacy of this C. capitata treatment for disinfestation of C. cosyra was thereafter confirmed in large scale trials in Valencia oranges. In the large-scale trial at the lowest mean temperature of 1.19 °C for 14 days, there were no survivors from a total of 85 490 treated C. cosyra third instars in oranges. Since C. capitata was shown to be more cold tolerant than C. cosyra and a large scale test demonstrated at least 99.9965% efficacy after 14 days at 1.19 °C, compared with the established effective C. capitata cold treatment of 14 days at 1.11 °C, cold disinfestation treatments for C. capitata should be at least equally effective against C. cosyra.