AbstractThe ultrastructure of mycoparasitic interactions in heterobasidiomycetes is diverse. A previously unknown cell structure, involved in a specific parasitic interaction, is described and illustrated. Cells of the parasitePlatygloea peniophorae(Auriculariales s.l., Basidiomycetes) which attach to host cells ofHyphoderma praetermissum(Aphyllophorales, Basidiomycetes) form distinct marginal vesicular bodies with electron‐opaque cores and electron transparent sheaths. Finally, the vesicular content projects through the cell wall of the parasite and then interacts with the cell wall of the host. Ontogenetic stages are described and illustrated in detai