ABSTRACTThe ultrastructure of dormant basidiospores ofPsilocybe cubensisis described. The spore wall is characterized by three distinct layers and a germ pore. A pore cap is described for the first time in a species ofPsilocybe.The protoplast is surrounded by a well defined plasma membrane with many distinct invaginations. Internally, large numbers of nonmembrane-bound lipids occur at both ends of the spore. Two nuclei are typically present and the nuclear envelope has many nuclear pores. Mitochondria with only a few, but well delineated, plate-like cristae are present. There is scant endoplasmic reticulum. Ribosomes occur regularly attached to the ER and outer mitochondrial membrane, as well as being densely packed throughout the cytoplasm. Variously sized vacuoles were demonstrated cytochemically to contain acid phosphatase. Microbody-like organelles are described but cytochemical tests to determine if these organelles are functionally similar to those of higher plants were unsuccessful. Cell-free extracts of basidiospores exhibit catalase and isocitrate lyase activity but no malate synthase activity.