Abstract The longevity of Drosophila melanogaster adults was measured in the absence of food and water and when maintained on a variety of diets. When starved from the time of emergence, or given only water, or only yeast or casein, the flies live but a few days. On a diet of glucose they live for some time because the oxidation of glucose permits the mobilization of food reserves laid down in pre-adult life. Flies kept on a diet of glucose and casein do not live as long as those which receive a yeast extract in addition, or as long as those kept on a medium fermented by live yeast cells. Flies free of micro-organisms live as long on an autoclaved medium containing sugar and yeast as those on a fermenting medium. Sterility was achieved using the antiseptic methyl p -hydroxybenzoate and the antibiotic chloramphenicol. These substances do not seem to be toxic when fed to Drosophila even at very high concentrations. It is concluded: (1) that a respiratory substrate must be included in the diet of adult Drosophila if maximal longevity is to be achieved; (2) yeast supplies nutrients in addition to essential amino acids; (3) there is no reason for believing that a diet rich in nutrients is life-shortening for Drosophila adults.