Competition and other factors influencing the population dynamics of Aphis gossypii and Macrosiphoniella sanborni on greenhouse chrysanthemums were studied. Single- and mixed-species populations after an accelerated growth period, reached a moderately stable equilibrium phase. Removal of alate forms, which simulated dispersal, reduced the rate of population growth, but did not stabilize the populations. Because they were strongly influenced by aphid density, the main population regulating agents were aphid size (related to birth rate), and leaf mortality (related to death rate). Another indirect, regulating factor was high temperature, which killed more of the older leaves when they were infested with high aphid densities. Although Myzus persicae is the major aphid pest of greenhouse chrysanthemums in California, this study suggests that, in the absence of insecticides, M. sanborni and A. gossypii are better adapted to chrysanthemums.