Electron microscopic examination and morphometric evaluation of Drosophila auraria larvae fed on diets rich in starch, casein or cholesterol for 24 h indicated that each of these nutrients influence, perhaps upon a secretagogue mechanism, the number of secretory granules which are produced in several midgut regions. The relation of these secretory granules to digestive enzymes as well as the participation of the midgut regions in the digestion of starch, casein and cholesterol is discussed.