Enchenopa binotata is a complex of six treehopper species that have diverged along host plant lines. When females were forced to oviposit on "adopted" host plants, few eggs were deposited. Fewer eggs hatched on "adopted" hosts and those that did hatch did so in response to the phenology of the "adopted" host. Mortality of nymphs on "adopted" hosts was substantially higher than on native hosts. These and other data support a sympatric model of speciation through shifts in host plants.