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Galls of the aphid Thecabius populimonilis were sampled from cottonwoods in Colorado, Utah and Arizona to investigate its life cycles. Both a monoecious and a heteroecious series of generations were found to occur in Colorado. First-instar nymphs deposited by emigrants were very similar to those of the sexuparous generation produced on the primary host, especially in having a characteristic set of wax plates on the abdominal tergites. This corroborated the generation-packing hypothesis that the monoecious series of generations was secondarily derived from the heteroecious series through the packing of the sexuparous generation into the gall. Further studies will be necessary to determine whether aphids exhibiting the two life cycles belong to the same or different species.