SUMMARYField‐caged, post‐anthesis populations of the aphidsSitobion avenaeandMetopolophium dirhodumreduced grain weight of wheat by 14 and 7% respectively and induced changes in the senescence of the flag leaf. Spikelet number and grain number were unaffected as they are normally determined by pre‐anthesis factors. Percentage grain protein was significantly reduced by both aphid species.The pattern of grain weight reduction within the ear was consistent with known limitations on the distribution of flag leaf assimilates among the grains. The relative effects of the two aphid species apparently resulted from the degree of nutrient drain imposed at particular feeding sites and the reduction in the leaf area duration of the flag