Plants of Vicia faba were grown in pots at photosynthetic photon flux densities (PPFD) of 143, 315 or 906 μmol m−1 for 8, 13 or 18 d. The effects of the dry matter loss caused by the feeding of Aphis fabae, or by removal of the cotyledons, were measured. Differences in dry weight between infested and control plants were positively correlated with the dry matter ingested, and increased with PPFD and time. Dry weights of aphid colonies increased non-linearly, but the honeydew produced per unit weight of aphid linearly, with PPFD. Aphid production/ consumption (P/C) ratios had a significant inverse linear relationship to log PPFD (r= 098). Net assimilation rates (NAR) calculated from the dry weights of the infested plants alone were significantly lower than in controls, but when calculated with the ingested dry matter added to the plant weight were little different from controls. No stimulations of NAR were found. Dry weights of all plant components, leaf area and specific leaf area were reduced by low light, removal of the cotyledons and by infestation. The last increased leaf area ratio (LAR) at the two later harvests but, otherwise, the treatments had only minor effects on the partitioning of dry matter in the plants.