Naturally grown and in vitro synthesized ectomycorrhizas of Hebeloma crustuliniforme and Picea abies are characterized by a whitish web-like mycelium on the surface which connects several ectomycorrhizal rootlets, a feature which seems independent of the host plant. Comparisons of naturally grown with in vitro synthesized symbioses showed that proportions of prosenchymatous and synenchymatous cells in the hyphal mantle vary, probably due various developmental stages. Rhizomorph formations have only been observed in natural symbioses. Ectomycorrhizas of pure cultured syntheses with seedlings are suitable for comparisons with naturally grown ectomycorrhizas since H. crustuliniforme forms symbioses in nature with young trees and therefore is considered as an 'early-stage' fungus.