A laboratory study of energy flow in different—aged pea aphids was accomplished by the balance sheet method. Growth, reproduction, molted exoskeletons, oxygen consumption, and honeydew production were monitored daily. Absolute amounts of energy expended on reproduction, growth, maintenance, and rejecta all varied with age of the aphid. Efficiencies of production and respiration also varied with the life history stage. Assimilation efficiency did not vary with age. Gross productions efficiency (° = 49%) and assimilation efficiency (°=83%) are higher in this insect than in most other insects reported. The reason for such high efficiency may be the unusually high amino acid content of pea phloem sap. Rates of exudation of amino acids and sugars from pea phloem through severed aphid stylets and greater than twice the total daily energy requirement of continuously feeding aphids. This would be true even for passively feedings aphids. Under the study conditions the daily ingestions of one average adult aphid is approximately 8% of the daily net primary production of a 0.15 a (dry wt) pea seedling.