• The short-term effects (24 h infestation) of the pea aphid on 14C-assimilate partitioning and stem elongation rate (SER) of alfalfa were investigated in relation to possible mechanisms (nutrient removal, mechanical or chemical stimuli) involved in the impact of the aphid on plants. • Different combinations of aphid numbers, developmental stages and location on the stem were tested on both SER and 14C-assimilate partitioning within the plant overall, and in the various compartments of the growth zone (GZ): apex, apical bud and elongating internodes. • Stem elongation rate reduction could be related quantitatively to assimilate withdrawal but did not depend on this parameter only. In the case of moderate aphid infestations located not only on but also below the GZ, the inhibition of assimilate allocation to the compartments of the GZ increased acropetally. The apex, a sink with mitosis and organogenesis activities, was dramatically affected. • These results are consistent with the development of an ‘inhibition-competition' mechanism resulting from reduced apical sink strength through the propagation of signals triggered by aphid feeding.