1. Methods are described for the aseptic culture of Drosophila.
2. All the factors necessary for normal development can be extracted from yeast water-soluble form.
3. A chemically defined medium sufficient for almost normal growth is described and includes: casein (and gelatin) as a source of amino-acids, dextrose, cholesterol, ergosterol, yeast nucleic acid, inositol, biotin, aneurin hydrochloride, riboflavine, nicotinic acid, pyridoxin hydrochloride, Ca-pantothenate, choline chloride, thymine and folic acid.
4. For completely normal development an alkali-soluble fraction must be added. This fraction shows nucleoprotein-like reaction, but it is neither arginine nor nucleic acid.
5. Folic acid is essential for pupation.
6. Attention is drawn to the need for caution in gene-action studies with synthetic media which do not give completely normal development even in the wild type.