Morphological plasticity is highly developed in the cotton aphid, Aphis gossypii. Adult aphids vary in size and in color, from yellow to green to black. Variation in body color is associated with several important aspects of aphid development and reproduction: dark morphs develop more rapidly, are larger, produce their progeny earlier during adult stage, and are more fecund. Dark aphids are therefore capable of much more rapid population expansion than are yellow aphids. Because only the dark aphids appear to generate outbreak populations capable of damaging cotton, we conducted laboratory experiments to investigate environmental factors that induce the production of this morph. Using genetically identical aphids from a single clone, we demonstrate that larger, more fecund aphids are produced under cooler temperatures, shorter day lengths (fall days), and on nutrient-rich plants. The hot weather during mod-summer in the San Joaquin Valley during most, but not all, years probably contributes to the prevalence of the yellow morph during the mid season. Cotton aphid management will need to focus on the highly reproductive dark morph.