Several authors have proposed that soils are made up of aggregates of various sizes, stabilised by different organic and inorganic materials. Fungal hyphae have been shown to bind microaggregates (<250 µm diameter) into macroaggregates (>250 µm diameter). This paper examines the aggregation of soil clay by saprophytic (Rhizoctonia solani and Hyalodendron sp.) and mycorrhizal (Hymenoscyphus ericae and Hebeloma sp.) fungi. The results support the hypothesis that fungal hyphae bring mineral particles and organic materials together to form stable microaggregates at least <2µm, and enmesh microaggregates into stable aggregates >50 µm diameter.