Under axenic conditions, larvae of Delia antiqua develop to the adult stage on onion agar only if the agar is supplemented with mixtures of 10 essential amino acids and 8 water-soluble vitamins. The omission of the amino-acid mixture allowed no development past the first instar. When the vitamin mixture was omitted, 1 of 25 larvae developed to the pupal stage. Omission of individual vitamin supplements caused less marked effects. Omitting supplemental choline, thiamine, pyridoxine, or pantothenic acid had no significant effect on development. Omitting supplemental biotin or nicotinic acid resulted in suboptimal development to the adult stage. Omitting riboflavin or folic acid resulted in development to the adult stage, but the adults did not lay viable eggs. Presumably nutrients that are in insufficient supply in onion tissue are supplied by bacteria in nature.