SUMMARYFruit bodies ofNidularia pulvinatawere grown in the laboratory on a special nutrient-supplemented straw medium. Fruit bodies were cryo-sectioned, fixed, dehydrated, critical-point dried, and observed in a scanning-electron microscope. Young fruit bodies were clearly differentiated into an upper portion and a lower portion where peridiole formation began. The peridium of young fruit bodies was composed of dense spinose skeletal hyphae which differentiated into three layers as they matured. The peridioles were formed from spherical aggregations of hyphae. They eventually developed a three-layered cortex. The peridioles gradually became oval with a central cavity in which basidiospores were produced. Four basidiospores on long sterigmata were formed on each basidium. Distribution of gelatinous material appeared to be correlated with development. Some similarities in ultrastructure betweenNidulariaandCyathuswere noted.