Technical Abstract: Culture-independent analysis of the gut of a wood-boring insect, Anoplophora glabripennis (Coleoptera: Cerambycidae) revealed that a fungal species, Fusarium solani, is consistently associated with the larval stage of this insect. Using the translation elongation factor 1-alpha region for phylogenetic and OTU-based analyses, the presence of F. solani was consistently detected in both colony derived and wild A. glabripennis populations, but genetic variance at this locus was low among A. glabripennis-associated isolates. To better characterize the genetic variation of F. solani associated with A. glabripennis and establish its phylogenetic relationship with other F. solani isolates, monoconidial cultures were created from different populations, and phylogenetic analysis was performed using a combination of the translation elongation factor alpha-1, internal transcribed spacer, and large subunit rDNA regions. These analyses revealed that colony-derived larvae reared in three different tree species or on artificial diet, as well as larvae from wild populations collected from four locations in New York City from three additional tree species, and from a single tree species in Worcester, MA, consistently harbored F. solani within their guts. While there is some genetic variation in the strain carried between populations, within-population variation is low. We speculate that F. solani is able to fill a broad niche in the A. glabripennis gut, providing it with fungal lignocellulases to allow the larvae to grow and develop on woody tissue. However, it is likely that many F. solani genotypes could potentially fill this niche, so the relationship may not be limited to a single isolate. While little is known about the role of filamentous fungi and their symbiotic associations with insects, this report suggests that larval A. glabripennis has developed an intimate relationship with F. solani that is not limited by geographic location or host tree.