TWENTY different species of insect larvae, distributed among four orders (Orthoptera, Coleoptera, Lepidoptera and Diptera) have been found so far to require sterols as growth factors. All of them, excepting the Dermestidae (Dermestes vulpinus Fabr.1 and Attagenus piceus Oliv.2), can utilize both zoosterols and phytosterols; certain species, however, grow faster with dietary cholesterol than with a phytosterol3. Furthermore, the activity of the intestinal sterol esterase of Periplaneta americana L. was found to be limited to the esters of cholesterol, Δ7-cholestenol and 7-dehydrocholesterol as the substrate4. These observations, together with Bergmann's5 report on the cholesterol content of the phytophagous silkworm Bombyx mori L., led us to suspect that the dietary phytosterols are converted to a zoosterol before utilization. Hence, sitosterol conversion was investigated in the larva of the housefly.