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All Rhizophagus spp. have antennal grooves on either side and slightly under the head (Bousquet, 2004). Body is fairly flattened, elongate and small, under 4.5 mm in total length (Bousquet, 2004). The final abdominal tergite (the last segment) is not covered by elytra (Sengupta, 1988). Larvae may be 2-6 mm in length and flat or cylindrical or tapered at both ends (Lawrence, 1991). Members of R. brunneus are distinguishable by their pronotum, the section of dorsal exoskeleton directly behind the head, being lined on both sides with a small lip. This also allows the centre of the pronotum to be slightly domed (Bousquet, 1990). The difference between the subspecies lies in the divets present on the pronotum. In R. brunneus brunneus the small divets are evenly distributed across the pronotum, whereas in R. brunneus fenyesi the divets are larger and more numerous down the centre of the pronotum (Bousquet, 1990). This species differs from R. remotus in that R. brunneus have semi-circular clubs where the 11th antennal segment is inside the 10th. In R. remotus the 11th segment is visible and the clubs are rounded (Bousquet, 1990).

Rhizophagus brunneus may play a role in transmission of Ceratocystis fimbriata, the fungus causing aspen canker on trembling aspen between trees (Hinds, 1972). Rhizophagus brunneus may be found in fungus dominated areas found on tree hosts (Hinds 1972) and has an affinity for decomposing cambium (Lawrence, 1991). Catch of this species in window traps in forested northern Alberta is most abundant from mid to late July (J. Jacobs, unpublished data). Contrary to the conifer hosts this taxon is usually collected on, these samples yielded large numbers of R. brunneus in conjunction with dying deciduous trees, indicating that this species may be more attracted to decaying material than bark beetles themselves.

Rhizophagus brunneus brunneus have been found under the bark of unspecified pine and spruce species and R. brunneus fenyesi are presumed to feed on similar hosts (Bousquet, 1990). In Alberta, R. brunneus have been found on white spruce (Picea glauca Moench) and unspecified spruce spp., sometimes in conjunction with bark beetles Dendroctonus americanum.

Rhizophagus brunneus are present in boreal or cold/temperate regions across North America (Bousquet, 1990). Rhizophagus brunneus brunneus Horn is present across Canada from Edmonton east through the Maritimes, collected specimens being clustered in the east, and south to Alabama (Bousquet, 1990). Rhizophagus brunneus fenyesi Méquignon has only been found in Arizona, New Mexico and northern Mexico (Bousquet, 1990).