Pupae of the Queensland fruit fly or Q-fly Bactrocera tryoni (Froggatt) are irradiated routinely to induce reproductive sterility in adults for use in sterile insect technique programmes. Previous studies suggest that adult sexual performance and survival under nutritional and crowding stress are compromised by the current target dose of radiation for sterilization (70-75 Gy), and that improved mating propensity and survival under stress by irradiated males may be achieved by reducing the target sterilization dose without reducing the level of induced sterility. This raises the question of the amount by which the irradiation dose can be reduced before residual fertility becomes unacceptable. The present study measures the levels of residual fertility in male and female irradiated Q-flies at different irradiation doses (20, 30, 40, 50, 60 and 70 Gy), and investigates the possibility that fecundity and fertility increase between 10-15 and 30-35 days post emergence. Male flies require a higher dose than females to induce sterility, with no residual fertility found in females irradiated at doses of 50 Gy or above, and no residual fertility found in males irradiated at doses of 60 Gy or above. Irradiated females are more fecund at 30-35 days post emergence than at 10-15 days. However, fertility does not increase between 10 and 15 days post emergence and 30-35 days, even at doses below 50 Gy. The present study shows that there is scope to reduce the target sterilization dose for Q-flies below that of the current dose range (70-75 Gy) at the same time as retaini ng an adequate safety margin above radiation doses at which residual fertility can be expected.