Transmission electron microscopy observations performed with cytochemical stains to detect polysaccharides and cysteine-rich proteins have been done to study the effect of an auxin transport inhibitor (2,3,5-triiodobenzoic acid, TIBA) on Laccaria bicolor (Marie) Orton. hypha attachment and aggregation during mycorrhiza formation in Picea abies (L.) Karst. roots. When the two partners were growing separately without any exchange of information, TIBA did not affect the cell wall's polysaccharide or protein structures, which could play a role in the aggregation or attachment process. The presence of the host strongly increased the production of fungal polysaccharide fibrils, allowing hypha aggregation and attachment with the roots. TIBA inhibited this host effect. Thus, we can hypothesize that TIBA, by preventing fungal indole-3-acetic acid (IAA) transport towards the root, inhibited the production or the efflux of host elicitors responsible for the increase of fungal polysaccharide fibril production. However, we cannot exclude that TIBA had other effects than inhibiting fungal IAA transport.Key words: ectomycorrhizas, auxin transport inhibitor, polysaccharide fibrils.