We characterized gut microbial communities in the emerald ash borer, Agrilus planipennis Fairmaire, an invasive phloem-feeding and wood-boring beetle that has caused extensive mortality to urban and forest ash trees. Analyses included both 16S rRNA gene–based and culture-based approaches. We estimated that the emerald ash borer gut harbors 44, 71, and 49 operational taxonomic units (OTUs_0.03) in the larval, prepupal, and adult stages, respectively, and a total of 132 OTUs_0.03 when data from the three stages are pooled. The larval gut community shared all its OTUs_0.03 with either the adult or the prepupal gut community, and the adult and prepupal gut communities shared 27 OTUs_0.03. Twenty-two OTUs_0.03 were shared among the three life stages. Rarefaction analyses suggest that these gut microbial communities are close to being completely sampled at the phylum level. Culture-independent techniques yielded a higher diversity of bacteria than did culturing. Three species of bacteria inhabiting guts of emerald ash borer showed cellulolytic activity. The diverse, dynamic, and presumably multifunctional microbial community associated with emerald ash borer guts suggests that invasive insects should be viewed as multispecies complexes and that such an interpretation can improve our ability to develop more effective management approaches.