Similar groups of apterous adults of the green peachaphid, Mysus persicae (Sulzer), and the larvae they depositedwere maintained, highly crowded, on pairs ofsynthetic diets differing solely by the omission of certainamino acids from 1 of a pair. The proportions of apteraethat developed among the larvae were found to be greatlyincreased when methionine, isoleucine, or histidine wereomitted. These 3 aminos acids have been shown bygrowth experiments to be essential amino acids. Thelessor effects of lysine, threonine, and cysteine may beassociated with the fact that although their omissionfrom synthetic, diet retarded growth, they could not beshown to be indispensable. Omission of alanine, tryptophane,or yaline tended to reduce the proportions ofapterae, while the remaining 10 amino acids had little orno effect. Growth experiments failed to show any deleteriouseffects when any of these latter 13 amino acidswere omitted singly from synthetic diet. One experimentusing a pair of diets with and without histidine showedthat differences in the proportions of apterae werebrought about by a direct effect of diet on young larvae,and indicated also that an indirect effect of the dietwas mediated to the larvae via their mothers. The bearingof these results on current theories of morph determinationin aphids is discussed.