A saprophytic, lignin-decomposing basidiomycete fungal isolate (BB1), identified as a member of the russuloid lineage closest to the genus Peniophora, plays a role in soil aggregation and stabilization. In liquid media this fungus secretes large amounts of extracellular materials or mucilage that act as soil binding agents. We investigated the nature of these materials using periodate treatment and lectin cytochemistry, and studied whether or not these materials are involved in soil aggregation. Water stability of artificial fungal amended soil aggregates was destroyed by periodate treatment suggesting that polysaccharides produced by the basidiomycete were involved in soil aggregation. Binding patterns of fluorescently labeled lectins were also investigated to determine specific carbohydrate moieties present in the fungal mucilage. Fluorescently conjugated lectins (Ulex europaeus Agglutinin I and Lotus tetragonolobus lectin) bound to extracellular mucilage indicating that this basidiomycete mucilage contains fucosyl sugar residues. We also demonstrated that water stability of artificial soil aggregates prepared with fungal mycelia pretreated with L(-)fucose lectins were significantly reduced. This study indicates that fungal-derived material containing fucosyl residues plays a role in soil adhesion.