Recent fears of terrorism have provoked an increase in delays and denials of transboundary shipments of radioisotopes. This represents a serious constraint to sterile insect technique (SIT) programs around the world as they rely on the use of ionizing radiation from radioisotopes for insect sterilization. To validate a novel X ray irradiator, a series of studies on Ceratitis capitata (Wiedemann) and Anastrepha fraterculus (Wiedemann) (Diptera: Tephritidae) were carried out, comparing the relative biological effectiveness (RBE) between X rays and traditional γ radiation from ⁶⁰Co. Male C. capitata pupae and pupae of both sexes of A. fraterculus, both 24–48 h before adult emergence, were irradiated with doses ranging from 15 to 120 Gy and 10–70 Gy, respectively. Estimated mean doses of 91.2 Gy of X and 124.9 Gy of γ radiation induced 99% sterility in C. capitata males. Irradiated A. fraterculus were 99% sterile at ≈40 – 60 Gy for both radiation treatments. Standard quality control parameters and mating indices were not significantly affected by the two types of radiation. The RBE did not differ significantly between the tested X and γ radiation, and X rays are as biologically effective for SIT purposes as γ rays are. This work confirms the suitability of this new generation of X ray irradiators for pest control programs that integrate the SIT.