New Zealand's endemic longhorned beetle larvae, the Huhu grub (Prionoplus reticularis) feeds on dead coniferous wood. No studies have been conducted on its gut microflora. Given that the Huhu grub feeds solely on lignocelluloses, it is likely that there are microorganisms present in its gut which are capable of degrading lignocelluloses to release energy rich sugars. This process of lignocelluloses release is the rate limiting step in the utilisation of woody material for bioprocesses such as bioethanol production. Microbial communities from wild grubs were compared with those raised on laboratory diets of either: lignocellulose, cellulose, or complex nutrients. Bacterial gut communities were surveyed using 454 pyrosequencing of the variable 5 and 6 regions (400nt reads) of bacterial 16S rRNA genes as well as clone library analysis of the full length gene (1500bp). Fungal gut communities were analysed using cloning and Sanger sequencing of amplified fungal intergenic spacer (ITS) regions. The wild type gut bacterial population was highly diverse, with no known cellulose or lignin degraders detected in any abundance, although a strain of Burkholderia thought to be capable of nitrogen fixation was detected. No methanogenic archaea or acetogenic bacteria were detected. Fungal ITS sequences had high similarity with those of known lignin, hemicellulose and cellulose degraders in the public databases, and an uncultured Basidiomycete made up 51% of the wild type community, while species of the Penicillium genus dominated the grubs reared on laboratory diets of lignocellulose. When grubs were reared on a diet of only cellulose the fungal community was dominated by a single species identified as Candida shehatae, a hemicellulose degrader known to associate with other longhorned beetle larvae. These fungi may be of interest for the biological conversion of lignocelluloses.