By comparing the growth of normal and aposymbiontic Lasioderma serricorne on diets of increasing cholesterol amounts, we demonstrated that the endosymbionts furnish sterols in quantities comparable to a cholesterol concentration of 0.01-0.03% in the diet (Fig. 1). We reared aposymbiontic larvae in diets without added sterol, which contained 50% of samples of different woods or cigarette tobacco. With fresh Aesculus wood and tobacco, larval growth was optimal. Other fresh woods (Acer, Prunus, Fagus, Quercus, Nothofagus) retarded development (Table 1). Old coniferous woods permitted almost no growth. In diets containing chloroform-extracted coigue wood (Sample No. 7), no growth occured. The diet of extract enabled the larvae to grow faster than that of unextracted wood. Thus, the larvae seem unable to utilize sufficiently the sterol content of wood. The ecological significance of these results and the importance of symbiontic microorganisms as sterol sources for insects are discussed.