The sporulation process in the thermophilic actinomycete Thermomonospora fusca was observed by scanning and transmission electron microscopy. As shown by scanning electron microscopy, spores were produced primarily on aerial hyphae and first appeared as bud-like enlargements at the tips of short multibranched sporophores. Young spores were oval to spherical in shape with a smooth surface. As they matured spores enlarged and developed a rough and globular covering, which was quite fragile and easily detached from the spore. This outer layer, as observed by transmission electron microscopy, was thought equivalent to the sheath of other Thermomonospora species. In cross section, mature spores exhibited a thick spore coat underneath the outer globular layer. This spore coat was usually observed as a single layer, but some spores produced a bilayered coat. No multilayered spore coat or spore cortex was observed in the heat-sensitive spores of T. fusca. They were, therefore, shown to be aleuriospores (microcondia), and not endospores.