The effect of gamma radiation on Nezara viridula male gonad cells was studied using transmission electron microscopy. When fourth instar nymphs were irradiated with a dose of 40 Grays by a ⁶⁰Co gamma unit, the gonads from the resulting adult males were abnormal in shape and colour compared with those of non-irradiated males. The ultrastructure of the developing spermatids and sperms from irradiated bugs was variable compared with non-irradiated bugs, which all appeared uniform in structure. In particular, there were structural abnormalities in the mitochondrial derivatives and axonemes. Further, supernumerary mitochondrial derivatives and axonemes were observed in the flagella of spermatids and sperms from irradiated males. In spite of these differences, some appear to be normal in structure. The results from high dose radiation-induced damage can be used to aid the development of the sterile insect technique (SIT) for control of N. viridula and other hemipteran pests.