Feeding by aphids during 11 days decreased plant weight between 4.7% at an initial density of 5 aphids/plant and 63.9% when the infestation began with 50 aphids/plant. At this infestation level, plant surface area was decreased by 54.3% after 10 days. Both soluble carbohydrate and total protein were reduced by aphid feeding but caloric equivalent of pea tissue was not significantly affected. Extraction by aphids of up to 3.5 cal/plant day was offset by a compensatory increase in production by the plant. Extraction of about 32.7 cal/plant day, equivalent to 0.46 adult aphids/cm2 of plant surface, completely eliminated pea net primary productivity. The relation between energy extracted by aphids, X, and % reduction of plant NPP, Y = 5.54 X−0.058 X2−19.17. Aphid consumption affected pea production directly by removing the amount actually ingested and indirectly because of lost production that would have resulted from the consumed material. Maximum sustainable efficiency of utilization of pea NPP by aphids was between 41% and 47% and maximum sustainable efficiency of transfer of pea NPP to aphid biomass was between 20% and 23%. These high utilization and transfer efficiencies probably would be reduced under field conditions by factors which limit aphid numbers.