1. A technique for sterilizing large numbers of Drosophila eggs is described. This gives about 95% successful cultures.

2. Using this method, dose-response curves have been obtained for all the main dietary constituents under conditions in which interactions between them are likely to be of little significance. Responses to the following are detailed: casein, fructose, cholesterol, lecithin, RNA, thiamine, riboflavine, nicotinic acid, pantothenic acid, biotin, pyridoxine, folic acid, choline and a water-soluble fraction of yeast which improves growth.

3. Other substances which have been included in Drosophila media are shown to have no value for the inbred strain of flies tested.

4. It is also shown that only K, P, Mg and Na salts need to be included in the food.

5. On the basis of these findings an optimal medium is formulated which allows larval development to be completed in about 4.4 days. This compares with 4.1 days when the same strain is reared on an optimal supply of killed yeast.

6. The minimal nutritional requirements of Drosophila larvae are shown to be considerably less than those of other aseptically cultured insects and, as far as the vitamins are concerned, only broadly related to the amounts found in yeast. The possible reasons for this are discussed.

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