The goal of this study was to characterize the cellulose-degrading microorganisms in the guts of wood-inhabiting beetles. We enriched for cellulose-degrading microorganisms by inoculating filter paper in liquid growth medium with macerated guts from larvae and adults of the wood-boring longhorned beetle, Saperda vestita, and the phloeophagous bark beetles, Ips pini and Dendroctonus frontalis. After incubation, microorganisms were isolated in pure culture and tested for their ability to digest carboxymethylcellulose (CMC). Isolates were identified based on their ribosomal RNA gene or intergenic region sequences. Bacteria and fungi from the gut of S. vestita degraded filter paper, whereas the microorganisms from the phloeophagous insects did not. The only bacterium detected that was able to degrade CMC is closely related to Sphingobium yanoikuyae, a member of the α-Proteobacteria class. We found this species in all field-collected S. vestita larvae in 2002 and 2003. Population densities of cellulolytic bacteria in S. vestita ranged from 2.4 × 105 to 3.6 × 106 CFU/gut. Bacteria isolated from the phloeophagous beetles I. pini and D. frontalis did not degrade CMC. Two fungi isolated from the gut of S. vestita adults had strong degradative activity. Sequences of the ITS1, 5.8S, and ITS2 rRNA regions indicated that these fungi are highly similar to Fusarium culmorum and Penicillium crustosum, respectively. This study provides the first description of the gut microbial community of S. vestita and the first documentation of association between cellulolytic microorganisms and a wood-boring beetle.