Aphis gossypii Glover and Acyrthosiphon gossypii Mordvilko (Hemiptera: Aphididae) are important pests of cotton (Gossypium hirsutum L.) in Xinjiang, China, that reduce yield and lint quality. We studied competition between the two aphid species in laboratory arenas to better understand population change and competitive advantage between the two and to provide a theoretical basis for the observed population outbreak of aphids in cotton fields. To study intraspecific competition, densities of 5, 10, and 15 aphids per 5-cm-diameter leaf disc were established in individual 6-cm-diameter arenas. Equal numbers of each species were placed on leaf discs in the arenas to establish densities of 3, 5, and 10 aphids of each species per leaf disc to assess interspecific competition. In intraspecific competition assays, the mean generation time (T) and the net reproductive rate (R_o) of both species decreased as aphid density increased, while the intrinsic rate of increase (r) and finite rate of increase (λ) of Acy. gossypii increased as density increased. In interspecific competition assays, population growth of A. gossypii was higher than with Acy. gossypii at the same density, while the mean generation time of A. gossypii was less than with Acy. gossypii. The net reproductive rate, intrinsic rate of increase, and finite rate of increase were higher in A. gossypii than in Acy. gossypii. Density is a key factor affecting competition between A. gossypii and Acy. gossypii. The higher the density, the more intense the interspecific competition, with interspecific competitiveness of A. gossypii stronger than that of Acy. gossypii.