Mediterranean fruit flies,Ceratitis capitata(Wiedemann), melon flies,Dacus cucurbitaeCoquillett, and oriental fruit flies, I),dorsalisHendel, were sexually sterilized with tepa supplied in drinking water for the first 2 days after adult emergence and at dosages determined in earlier tests to be thresholds for sexual sterilization of the particular species. Ovaries and testes were dissected out of the adults at different periods, depending upon the physiological ages of the particular species, and measurements were made under a binocular microscope with calibrated occular. Sizes of gonads of the males of all three species were affected less severely by the tepa treatment than those of females. Male and female gonads of treated Mediterranean fruit flies were least reduced in size, and those of melon flies the most.