SINCE our earlier publication1, the following additional information has been obtained about the nutritional requirements of Drosophila melanogaster. Addition of 25 γ thymine per culture to the basal medium leads to accelerated development, pupae being formed seven days after egg-laying. This represents a decrease in developmental time of one day at least over controls without thymine. Further acceleration can be obtained by addition of 20 mgm. of a yeast fraction prepared by treating yeast with hot 2 per cent sodium bicarbonate, filtering, rendering slightly acid with hydrochloric acid, and precipitating the fraction with an excess of ethanol. Pupation can then ensue five days after oviposition. That the active fraction is not nucleic acid as supplied commercially is shown by the fact that parallel cultures without the fraction, but containing additional nucleic acid, do not show the same effect.