[Objectives]  To determine the effects of density on the degree of interspecific competition between Aphis fabae Scopoli and Acyrthosiphon pisum (Harris), two important pests of leguminous crops that are usually sympatric on a same crop. [Methods]  The effects of different numbers of heterogonous species on the development and reproduction of A. fabae and A. pisum were determined, as was the effect of densities of 1, 5 or 10 A. fabae on A. pisum, and vice versa. In addition, the effects of pre-infestation by one species on the reproduction of the other were measured. [Results]  The developmental time of each instar and the duration required for 1st instar-to-adult of A. fabae and A. pisum were similar to those of the control (no competition) irrespective of whether there was one, or 5, heterogonous species present. When only a single heterogonous species was present, the weights of adult A. fabae and A. pisum were not significantly different from those of the control; however, when 5 heterogonous species were present the weights of adult A. pisum were significantly lower than the control and the weights of A. fabae were significantly higher than control. Reproduction of A. fabae was not significantly changed when 5 A. ptisum were present, whereas the presence of 5 or 10 A. fabae adversely affected the reproduction of A. pisum. When sympatric on live plants, first day nymph production of A. pisum was significantly lower than the theoretical value, whereas that of A. fabae was higher than the theoretical value. Pre-infestation by a heterogonous species caused reproduction of A. pisum to be significantly lower than that of the control, whereas reproduction of A. fabae was higher than that of the control. [Conclusion]  A. fabae is competitively superior to A. pisum. These results lay a solid foundation for further evaluating interspecies succession of these two aphids on leguminous crops, and for designing effective methods to control them.