Transmission electron microscopy was used to study the development and structure of resting spores and sporosori of Polymyxa graminis. Osmiophilic bodies generally distributed in the cytoplasm moved to the periphery as spores matured. The spore wall consisted of four or five layers, two of which extended into spines linking adjacent spores. An unstained plug in the wall, faced towards the outside of the sporosorus. Some spores germinated in situ by producing primary zoospores. Bundle-like structures in some spores may have been barley yellow mosaic virus particles.