Exposure of 36- to 48-hour-old pupae of the common house fly, Musca domestica, L., to single doses of X rays ranging from 2000 to 8000 rads had no appreciable effect on the percentage of emergence or on the normal appearance of the adult flies. All levels of radiation produced only a slight but significant decline in female life span. Such low levels of X-irradiation produced only a slight effect on male life span, with a decrease for 2000 rads, no effect at 4000 rads, and a slight increase at 6000 and 8000 rads. However, the effect of such X-irradiation on the otherwise normally high degree of wing loss in male house flies was much more striking, the number of male house flies retaining their wings at the time of death being trebled for all four dose levels studied. In the females, there was a smaller but nevertheless significant increase in such wing retention.