The goldenrod fly (Eurosta solidaginis ) is an herbivorous gallmaker found in prairie and forest habitats in the Upper Midwestern United States. The flies attack two species of goldenrod, Solidago altissima and S. gigantea. A tumor-like growth (gall) is formed by the host plant after the female fly deposits eggs on the bud. The galls are subject to predation from birds and other insects, a beetle and two parasitic wasps. This interaction has been used to demonstrate balancing selection and other important evolutionary concepts. The question remains, however, whether the fly is one or two species, or one species with two races. A previous study suggests that the flies are undergoing speciation driven by the female fly’s selection of either S. altissima orS. gigantea. A competing hypothesis is that the fly is undergoing speciation as the result of habitat selection (prairie versus forest) rather than host plant selection. Our objective is to use genetic data to clarify the identity of E. solidaginis. We will fully characterize the mitochondrial DNA genes COI and COII for the goldenrod fly. The results of this study will be valuable to researchers worldwide as E. solidaginis is a model organism in evolutionary studies involving speciation, selection, and evolution of host plant interactions.