The symbiotic bacteria Buchnera sp. provide aphids with essential amino acids, nutrients in short supply in the aphid diet of plant phloem sap. The contribution of Buchnera-derived amino acids to net protein growth of the aphid Aphis fabae was quantified from the protein growth of aphids reared on chemically defined diets lacking individual amino acids. The amino acid production rates varied among the nine essential amino acids over the range 8–156 pmol microg(−1)protein day(−1) (for tryptophan and leucine, respectively), equivalent to 0.02-0.33 fmol Buchnera(−1)day(−1). In a complementary metabolic analysis, the aphids incorporated radioactivity from dietary [(14)C]glutamic acid into the essential amino acids isoleucine, lysine and threonine. Incorporation into isoleucine was significantly elevated by the omission of dietary isoleucine, indicating that dietary supply may affect the biosynthetic rates of certain amino acids by Buchnera. Aphids experimentally deprived of Buchnera did not synthesize essential amino acids from dietary glutamic acid. The mortality of aposymbionts was high over 7 days on the phenylalanine-free diet, and their assimilation of dietary leucine was depressed on the complete diet, suggesting that both the absence of bacteria-derived amino acids and the low rates of assimilation of certain dietary amino acids may contribute to the poor growth of these insects.