Technical Abstract: The combination of heat and low levels of oxygen increases mortality to insects infesting fruit compared with either heat or low oxygen alone. This combination treatment shows promise to disinfest commodities of quarantine pests. Heated air/modified atmosphere treatments employ the modified atmosphere (e.g., low oxygen) during the entire treatment interval. There is a positive relationship between temperature and efficacy of heat/modified atmosphere treatments. Efficacy of delaying atmospheric modification in a heat/modified atmosphere treatment was studied with the Mexican fruit fly, Anastrepha ludens (Loew), (Diptera: Tephritidae), a quarantine pest of citrus and other fruits in Mexico, Central America and southern Texas. The fly was subjected to heat/low oxygen treatments in vitro as well as in grapefruit, Citrus paradise Macfayden. The relationship between time delay of the modified atmosphere and time required to kill 99% of Mexican fruit fly third instars was not linear, but indicated that mortality rate was greater later in the treatments when temperatures were higher. When infested grapefruits were heated with 47 degrees C air in 3 atmospheric regimes: 1) air, 2) N(2) at 99 kPa plus O(2) at 1 kPa, or 3) air for 55 min then N(2) at 99 kPa plus O(2) at 1 kPa for the remainder of the treatment, estimated 99% prevention of pupariation was 157, 127, and 141 minutes, respectively, with the latter treatment showing an intermediate level of mortality between the other two.