Nuclear ribosomal DNA sequences characteristic of Sebacinaceae were detected by direct amplification of DNA from field collected ectomycorrhizal samples. A study was undertaken to confirm the formation of ectomycorrhizas by Sebacinaceae and to characterise representative samples genetically, morphologically and ultrastructurally. The investigated sebacinoid mycorrhizas were sufficiently characteristic to be identified morphologically. They are distinguished by a characteristic, clampless, hydrophilic extramatrical mycelium, which is very variable in diameter and in wall thickness, and by the presence of distinctive, y-shaped, inflated multibranchings. Differences in the mantle structure clearly discriminate the two investigated sebacinoid mycorrhizal types. Comparison of the D1/D2 domains of the nuclear large subunit pseudogene and the ITS1 and ITS2 regions identifies the fungal partner of one of the mycorrhizal samples as Sebacina incrustans. Ultrastructural investigations of the ectomycorrhizas show a doliporus/parenthesome architecture consistent with that of the Sebacinaceae. Recently published sequence data obtained from sebacinoid mycorrhizas are compared to our sequences and the complex trophic relationships in the Sebacinaceae are discussed. Observations on ectomycorrhizas and basidiomes suggest that species of Sebacinaceae are fairly common mycobionts in various ectomycorrhizal plant communities.