A synthetic food medium for the cultivation of Drosophila melanogaster is described, and results of its use are compared with standard banana medium. The synthetic medium S-99 is composed of salts, sugars, and agar. It is shown that the H-ion concentration of the medium increases with age of the culture until a point of buffered equilibrium is reached, in cultures of Drosophila melanogaster, on both banana and a synthetic medium. The total acid per gm. of medium increases throughout the life of the culture as long as relatively large numbers of flies are present. The increase of total acid in cultures on banana medium follows an exponential curve y = 0.183e0.355:r, suggesting that the reaction is auto-catalyzed, while on the synthetic S-99 medium the increase of total acid follows a straight line. The fertility was higher on the synthetic medium, suggesting that it is better suited to practical fly husbandry than is the standard banana medium, and demonstrating that the presence of fresh banana, or any other fruit, is in no wise biologically essential as a constituent of the food medium in the culture of Drosophila so far as concerns the processes of reproduction.