A new synthetic food medium, S-101, for the cultivation of Drosophila melanogaster is described, with the following composition in grams Solution A: cane sugar, 500, KNaC4H4O64H2O, 50, (NH4)2SO4, 12, MgSO47H2O, 3, CaCl2, 1.5, H2O to make 3000 cc. of solution; Solution B: Agar-agar, 135, tartaric acid (C4H6O6), 30, KH2PO4, 6, H2O to make 3000 cc. of solution. Dissolve agar in hot water, add the salts, and for the medium to be used in the fly bottles, mix equal parts of solutions A and B. For some kinds of work it is desirable to have the final food a little less stiff, in which case a small amount of agar is used, without otherwise changing the composition. This medium when freshly made, cooled, and set has a pH of approximately 3.7. As the flies live upon it the pH falls to 3.0 or even lower. On this medium the fertility of Drosophila is uniformly greater than on standard banana medium, by amounts ranging from 47.7% increase of fertility at a population density of 2, to 98.0% increase at population density 64. In the 1st 8 days of life the mortality is only 1 /3 as great on the S-101 medium as on standard banana medium.