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The nests of mound-building ants are unexplored reservoirs of fungal diversity. A previous assessment of this diversity in the nests of Formica ulkei suggested that water availability may be a determinant of the composition of this mycota. To investigate this question, we recovered 3594 isolates of filamentous Ascomycota from the nests of Formica obscuripes and adjacent, non-nest sites, employing Dichloran Rose Bengal agar (DRBA), Dichloran Rose Bengal agar containing glycerol (DRBAG), and malt extract agar containing sucrose (MEA20S). Higher numbers of fungi were isolated from the tops of mounds than from within mounds and non-mound sites. Mound nest soils were dominated by members of the family Aspergillaceae, and up to 50% of the colonies isolated on DRBAG belonged to the genus Aspergillus. Pseudogymnoascus pannorum and species of Talaromyces were also present in higher numbers in mound soils. Species of Penicillium were more abundant in non-nest soils, where they accounted for over 66% of isolates on DRBA. All Aspergillaceae assessed for xerotolerance on a medium augmented with glycerol or sucrose were xerophilic. These results, and our observation that the nests of F. obscuripes are low-water environments, indicate that water availability influences the structure of the fungal communities in these animal-modified habitats.