Electron microscopy was used to compare four closely related genotypes of Laccaria bicolor (Maire) Orton (the two sib-monokaryons m4 and m10 having been crossed twice to give the two dikaryons d1 and d2) which showed substantially different ectomycorrhizal structures on roots of Pinus banksiana Lamb. All four strains incorporated into mucilage on long roots but only the most compatible strain dl formed thick mantles with closely packed hyphae at surfaces of both long and short roots. At the inner Hartig net found in short roots, hyphae of dl showed a highly convoluted morphology whereas other strains formed roundish hyphal cross sections. The rare Hartig nets formed by m4 occasionally showed highly distorted root cell walls adjacent to hyphae. The occurrence of extracellular glycofibrils during initial hypha-root contact was rare for all four fungal strains whereas substantial tannin-like deposits were frequently found in root cortical cells.