The cotton fleahopper, Pseudatomoscelis seriatus (Reuter), is one of the most important insect pests of cotton in the south-western United States. The cotton fleahopper attacks cotton in the early fruiting stage causing flower buds (squares) to abort. Although it is believed that fleahoppers move to cotton from wild weed hosts, which are abundant around cotton fields in early spring, the ecological consequences of such movements have not been clearly understood. More specifically, fleahoppers associated with different host plant species could belong to reproductively isolated populations. We have used amplified fragment length polymorphisms (AFLP) to verify the occurrence of reproductively isolated fleahopper populations associated with horsemint, Monarda punctata (a common weed host of the cotton fleahopper in central and west Texas), and cotton. Knowledge generated by this study will aid in our understanding of fleahopper movement among hosts and in formulating future management strategies.