Commensal microorganisms have significant impacts on the health of many insect hosts. Little is known, however, about the structure of commensal bacterial communities associated with the Cerambycidae, despite the important roles this large family of herbivorous endophytic insects plays in ecosystem processes, economic losses to ornamental and forest trees, and biological invasions. We analyzed the bacterial commensal communities of the exotic Asian longhorned beetle, Anoplophora glabripennis, and the native linden borer, Saperda vestita, by randomly sequencing 16S rRNA gene fragments from bacterial DNA extracted directly from the gut of larvae. The 16S rRNA gene sequences sampled from S. vestita were derived entirely from the γ-Proteobacteria phylum of Bacteria. In contrast, the gut of A. glabripennis larvae contained members of the α-Proteobacteria, β-Proteobacteria, γ-Proteobacteria, Firmicutes, Actinobacteria, and Bacteroidetes. A better understanding of insect-microbe interactions may lead to new strategies to reduce the effects of these pest species.