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Although Myzus persicae is the major aphid pest of chrysanthemums grown in commercial greenhouses in this area, this study suggests that in the absence of insecticides Macrosiphoniella sanborniand Aphis gossypii are better adapted to chrysanthemums. Because Myzus persicee was so poorly adapted to chrysanthemums, the study emphasizes the effects of alate production, aphid size, distribution on the plant, and inter-and intraspecific competition on population regulation of M. sanborni and A. gossypii. When Mac1•osiphoniella sanborni and Aphis gossypii were grown together, the stems and young leaves were preferred by M. sanborni, and young leaves and terminal buds were preferred by A. gossypii. Both species preferred the lower surfaces of the older leaves. Singleand mixed-species populations of Macrosiphoniella sanborni and Aphis gossypii, after an accelerated growth period, reached a relatively stable equilibrium phase. The species that attained numerical superiority during the growth phase tended to maintain this advantage. Removal of alate forms, which simulated dispersal, reduced the high rate of population growth, but did not stabilize the populations. Because they were strongly influenced by aphid density, the main population regulating agents were (1) aphid size (related to birth rate) and (2) leaf mortality (related to death rate). Another indirect, regulating continued on inside back cover THE AUTHORS: George Tamaki was a graduate student in Entomology and Parasitology, Berkeley, during this study and is now Entomologist,