Xylophagous insects often thrive on nutritionally suboptimal diets through symbiotic associations with microbes that supplement their nutritional requirements, particularly nitrogen. The wood-feeding cerambycid Anoplophora glabripennis (Motschulsky) feeds on living, healthy host trees and harbors a diverse gut microbial community. We investigated gut microbial contributions to larval nitrogen requirements through nitrogen fixing and recycling (urea hydrolysis) processes, using a combination of molecular, biochemical, and stable isotope approaches. Genes and transcripts of conserved regions of the urease operon (ureC) and nitrogen fixing (nif) regulon (nifH) were detected in A. glabripennis eggs and larvae from naturally infested logs and from larvae reared on artificial diet. Significant nitrogen fixation and recycling were documented in larvae using ¹⁵N₂ gas and ¹⁵N-urea, respectively. Subsequent ¹⁵N-routing of incorporated recycled nitrogen into larval essential and nonessential amino acids was shown for ¹⁵N-urea diet-fed larvae. Results from this study show significant gut microbial contributions to this insect's metabolic nitrogen utilization through nitrogenous waste product recycling and nitrogen fixation.