SUMMARYCyathus stercoreuswas grown in the laboratory and basidiocarps produced in culture were prepared for study with the scanning and transmission electron microscopes. The peridium consisted of three morphologically distinct layers. The funiculus consisted of three regions. The hyphae in these regions, the funicular cord and hapteron showed a special modification in the clamp connections. The peridioles had a wall consisting of three layers. The spores were surrounded by special nurse hyphae.Cyathus stercoreusstructure is far more complex than light microscopy has revealed in the past. Ultrastructural study shows the nature of some previously described structures to be in error.