The diet proposed by Hill et al. ('47) for P. regina was modified by substituting crystalline cholesterol for lanolin. The casein content of the standard diet was high (29%); however, replacement of a part of the casein by carbohydrate resulted in slower growth and metamorphosis. This effect progressed in the direction starch < sucrose < glucose, with complete growth failure on a diet containing 10% of glucose. Neither enzyme-hydrolyzed casein nor acid-hydrolyzed casein plus tryptophan was able to replace the whole casein in the diet. It was demonstrated that 0.83 mg of D-serine per gram of medium caused complete inhibition and resulted in early death of the small immature larvae. This inhibition could be completely reversed by 5 mg of D-alanine. The free amino group appeared to be required in both amino acids to evoke this response. A chemically-defined diet was prepared on which the blowfly developed from egg to adult. Ten amino acids proved to be essential: arginine, histidine, isoleucine, leucine, lysine, phenylalanine, proline, threonine, tryptophan, and valine. Six vitamins were shown to be necessary in the diet: thiamine, riboflavin, pyridoxine, nicotinic acid, pantothenic acid and choline. The possibility was considered that the developing larvae are sensitive to high osmotic pressure in the medium.