When the CSMA larval medium was supplemented with cholesterol, viable egg production from adult house flies (Musca domestica L.) fed on a sterol-deficient synthetic diet was about double that found for flies reared on the nonsupplemented medium. In contrast, there was in) increase in viable egg production when β-sitosterol was used to supplement the medium. Analysis of female house flies showed no correlation among concentration of dietary sterol, sterol content of the flies, and viable egg production, which indicated that a qualitative rather than a quantitative change in the sterol content of the medium was responsible for the observed phenomenon. This qualitative difference was further substantiated when cholesterol and β-sitosterol were compared in the adult diets.