When 2-C14-mevalonate was injected into male and female house flies (Musca domestica L.) at 10 micrograms per fly, about the same amount of radioactivity was incorporated into the saponifiable and unsaponifiable lipids after 18 hours. Fractionation of the unsaponifiable material by column chromatography demonstrated that less than 17% of the radioactive material behaved as hydrocarbons, and more than 40% was eluted in the sterol fraction. However, when this fraction was analyzed by digitonin precipitation, only trace amounts of radioactivity were precipitated with the sterol digitonides, indicating that the previously reported absence of sterol synthesis from C14-acetate in the adult house fly is not due to a metabolic block in the biosynthetic pathway between acetate and mevalonate.