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Nitrogen (N) fixation is a driving force for the formation of symbiotic associations between N 2 -fixing bacteria and eukaryotes. 1 Limited examples of these associations are known in fungi, and none with sexual structures of non-lichenized species. 2–6 The basidiomycete Guyanagaster necrorhizus is a sequestrate fungus endemic to the Guiana Shield. 7 Like the root rot-causing species in its sister genera Armillaria and Desarmillaria , G. necrorhizus sporocarps fruit from roots of decaying trees (Figures 1A–1C), 8 and genome sequencing is consistent with observations that G. necrorhizus is a white-rotting decomposer. This species also represents the first documentation of an arthropod-dispersed sequestrate fungus. Numerous species of distantly related wood-feeding termites, which scavenge for N-rich food, feed on the mature spore-bearing tissue, or gleba, of G. necrorhizus. During feeding, mature spores adhere to termites for subsequent dispersal. 9 Using chemical assays, isotope analysis, and high-throughput sequencing, we show that the sporocarps harbor actively N 2 -fixing Enterobacteriaceae species and that the N content within fungal tissue increases with maturation. Untargeted proteomic profiling suggests that ATP generation in the gleba is accomplished via fermentation. The use of fermentation—an anaerobic process—indicates that the sporocarp environment is anoxic, likely an adaptation to protect the oxygen-sensitive nitrogenase enzyme. Sporocarps also have a thick outer covering, possibly to limit oxygen diffusion. The enriched N content within mature sporocarps may offer a dietary inducement for termites in exchange for spore dispersal. These results show that the flexible metabolic capacity of fungi may facilitate N 2 -fixing associations, as well as higher-level organismal associations. [Display omitted] • The genome of Guyanagaster necrorhizus shows it is a white-rotting decomposer • N 2 -fixing bacteria increase the nitrogen content of its sporocarp tissue • The sporocarp environment is likely anaerobic to protect the nitrogenase enzyme Few examples of N 2 -fixing associations are known in fungi. Koch et al. show that sporocarps of Guyanagaster necrorhizus harbor N 2 -fixing bacteria, which benefit from the anaerobic environment provided within the sporocarps. The increased N content of sporocarps likely serves as a nutritional reward to termites who serve as the dispersal agent for this fungus. [ABSTRACT FROM AUTHOR]