Screw-worm (Callitroga hominivorax (Cqrl.)) larvae were reared in sufficient numbers to produce 2 million sterilized flies weekly for 15 consecutive weeks in Il Florida field test. Five days each week eggs were obtained from 20 colony cages, each stocked with 700 eight-day-old flies, by heating an attractive medium in the cages. Single cultures of 100,000 larvae were reared on a mixture of ground lean meat, blood, and water, in vats. Until irradiation as 6-day-old pupae, the insects were held at 80° F. and 90+ per cent relative humidity.