Ectomycorrhiza morphogenesis involves the differentiation of structurally specialized fungal tissues, the mantle and the Hartig net, and interfaces between symbionts. Proteins whose biosynthesis is specifically up- or down-regulated have been identified in several ectomycorrhizal associations. Their spatial and temporal expressions have been characterized by cell fractionation, two-dimensional polyacrylamide gel electrophoresis and lmmunochemical assays in the Eucalyptus spp -Pisolithus tinctorius symbiosis. These biochemical investigations have revealed the drastic alterations experienced by fungal cell wall polypeptides during the early stages of the ectomycorrhizal interaction. These include the increased synthesis of 32 kDa acidic polypeptides, together with the decrease in content of a major 95 kDa-mannoprotein, gp95. Differential screening of a mycorrhiza cDNA library and large-scale cDNA sequencing have allowed the characterization of several symbiosis-regulated fungal genes coding for cell wall proteins (e.g. hydrophobins). In P. tinctorius, the content of hydrophobin transcripts is up-regulated in aerial hyphae and during the mantle formation. The cell wall of the fungal partner therefore presents a novel protein composition, characteristic of the symbiotic status. A convergence of biochemical and molecular results now allows formulation of molecular models for the key role of the cell wall compartment in the early developmental stages of ectomycorrhiza formation. Clearly, we are only beginning to understand the formation of the symbiotic cell walls, their regulation and topologic specificity. Definitive information should emerge from biochemical, cytological and molecular analyses of the appropriate models.