Using Muller''s x-ray dosage, a study was made of the daily egg production in irradiated flies; mortality rate in eggs, larvae, and pupae; differences appearing in the sex ratio; and visible mutations. When the treatment is heavy, 77.3% of all eggs die in the egg stage. After light irradiation 58.3 % die. Of the eggs that hatch into larvae, 50.4% die in that stage or in the pupa in the heavy treatment group; 32.1% in the light treatment group. A slight effect upon the sex ratio is shown. Among the adult flies, 50 visible mutations were found; some of these are described and the percentage rate of mutation given.