<p>The qualitative and quantitative characteristics of microscopic fungi communities were investigated in soils along a toposequence from a floodplain to the summit of a calcareous plateau in the mountain landscape of the Polar Urals. The biomass of fungi in the studied soils varies between 0.13 ± 0.01 and 1.63 ± 0.81 mg/g. The main contribution to its structure comes from fungal spores (up to 100% of the total biomass). The length of mycelium of microscopic fungi in upper organic horizons of soils varies from 18.45 ± 8.70 to 162.71 ± 134.89 m/g. The total taxonomic list of microfungi includes 38 species from 19 genera, two divisions, and sterile mycelium. The smallest number of species (12) was recorded in the soil under the herb–sedge–moss community in the lower part of the slope, and the largest number of species was recorded in the soils of the floodplain grass–forb meadow and dryad patterned-ground tundra (18 species each). The Mucoromycota division is represented by 10 species from <i>Linnemannia</i>, <i>Mortierella</i>, <i>Mucor,</i> and <i>Umbelopsis</i> genera. Fungi of the Ascomycota division predominate in fungal communities of the studied soils on calcareous rocks. The <i>Trichoderma</i> genus (8 species) is the leader by the number of species. The <i>Penicillium</i> genus has low species diversity in conditions of weakly acidic and weakly alkaline pH. It has been shown that the fungal complexes of soils developed on calcareous rocks are characterized by low abundance, poor species and genus diversity, and the dominance of sterile mycelium. In tundra conditions, the quantitative characteristics of the fungal complexes in carbonate-containing soils depend on the composition and structure of the plant community as well as on the organic matter content in soil. While the CaCO₃ content does not fully determine the total abundance and biomass of fungi in soils, it may influence the taxonomic structure of fungal communities through an increase in pH.</p>