The development of ectomycorrhizas of pine was followed experimentally using either sterile peat–vermiculite with nutrient solution or natural non-sterile field soil. Roots were harvested sequentially and comparative analyses of their ultrastructure were obtained. Hartig net formation is initiated before the formation of a compact sheath in both types of synthesis but, while the net formed after 3 weeks in sterile culture and was complete after 6 and 8 weeks, in non-sterile soil its initiation was delayed until 11 weeks and it had reached a depth of only one cortical cell after 19 weeks. While differences in gross morphology of mycorrhizas can be recognized, the structure of the host–fungus interface in the Hartig net region is comparable in mycorrhizas formed with different soils or different isolates. In the young Hartig net, separate host and fungal cell walls are distinguishable, while in the mature net they lose their integrity. No marked change of structure is observed within the cortical cells adjacent to the Hartig net and their organization is similar to that of cortical cells of non-mycorrhizal lateral roots. The development and structure of the host–fungus interface is discussed in relation to its possible function.