Basal hymenomycetes belonging to the Sebacinaceae are ectomycorrhizal on temperate deciduous trees.

• Heterobasidiomycetous species of the Sebacinaceae family, previously considered as saprophytes or parasites, are shown here to form ectomycorrhizas on temperate forest trees. • Ectomycorrhizas were collected under sebacinoid sporophores and near root systems of Neottia nidus-avis, an orchid symbiotic with sebacinoids. To identify the partners each ectomycorrhiza was submitted to amplification and sequencing of the plant and fungal internal transcribed spacer (ITS), and further investigated by light and electron microscopy whenever a sebacinoid ITS was found. • Molecular and microscopic analyses correlated well. Two sebacinoids of divergent rDNA sequences were demonstrated to form similar ectomycorrhizas, with a well-developed Hartig net and a hyphal mantle having thick-walled outer mantle hyphae. The ultrastructure of the septal pore (dolipore with imperforate caps) was typical for sebacinoids. In one case, intracellular colonization was seen. The ectomycorrhizal host range of these sebacinoids was not specific and included Betulaceae, Fagaceae and Tiliaceae. • Sebacinoids probably represent an overlooked ectomycorrhizal group and ectomycorrhizal symbiosis may be common among basal lineages of hymenomycetes.