The effect of cholesterol deficiency on survival and reproduction of the adult house fly (Musca domestica L.) was investigated. The preparation and composition of the synthetic diet used in these studies are reported. Although a cholesterol deficiency had no appreciable effect on adult survival or total egg production, it caused nearly an 80% reduction in egg hatch. Only about half the eggs that hatched from the flies fed the steroldeficient diet produced adults when reared on a larval medium containing sufficient sterols to support growth. Of the various cholesterol concentrations tested, 0.1% gave the highest cumulative hatch (92%) and a total viable egg production about 30% greater than that found for the control diet (sucrose: defatted dry milk solids 1:1).