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Some herbivorous insect taxa that exhibit otherwise conservative patterns of host association include species or populations with atypical host specificities. These species provide special opportunities to study the evolution of host range. Uroleucon ambrosiae aphids present an example. Most Uroleucon species are monophagous on particular host plants from the Asteraceae, and U. ambrosiae appears to specialize on the giant ragweed, Ambrosia trifida, in eastern North America. In the American Southwest, however, U. ambrosiae uses a variety of asteraceous taxa as hosts. For the present study, we assayed host-associated behaviors of U. ambrosiae from both eastern and southwestern regions of the United States on each of four asteraceous genera. Data from choice and no-choice experiments and electrical penetration graph analyses revealed highly significant differences in the acceptability of the four test plants. Plant taxa were ranked in the same order across multiple behavioral assays by aphids from both regions. However, eastern and southwestern aphids exhibited significantly different patterns of behavior. Ambrosia was the most highly accepted plant by aphids from both regions, but southwestern aphids accepted the other test plants more readily than did eastern aphids, indicating geographic variation in degree of host specificity. This regional differentiation held for aphids reared under controlled conditions for the EPG studies, indicating a genetic basis for population divergence in U. ambrosiae host-use traits. We speculate that the generalism within U, ambrosiae is an evolutionarily derived trait and represents an ecological adaptation to the scattered and unpredictable distribution of the preferred A. trifida host in the arid American Southwest.