[en] A technique has been developed to distinguish between irradiated (sterile) and normal (wild or laboratory) sperm found in spermathecae of mated Mediterranean fruit flies, Ceratitis capitata (Wiedemann). Darkfield microscopy (l,000 × magnification) can be used to separate readily the sperm head lengths of irradiated males (20.5-29.0 μm; mean, 25.7) from those of wild males (27.5-33.0 μm; mean, 30.1). The sperm heads of laboratory adapted, nonirradiated males are intermediate in length compared with sperm from sterile and wild individuals. Sperm heads from three different wild populations in Hawaii were all strikingly similar in length. Field-collected females can be dissected to assess the mating frequency of released males in a sterile insect technique program. The frequency of multiple matings involving both types of males can also be measured. Two Bactrocera species found in Hawaii were examined and were found to have very similar sperm-head lengths when compared with each other, yet each had ≍3 μm longer average sperm-head lengths compared with those of C. capitata. The technique may be further useful in distinguishing sperm among other interspecific or conspecific tephritid populations. The potential effects of varying laboratory rearing or irradiation conditions on sperm-head length in tephritids remain to be investigated