Summary: Thin sections, freeze-etch preparations and carbon replicas of Thermoactinomyces vulgaris sporangia and endospores were examined in the electron microscope. The outer spore coat contained parallel arrays of long fibrous striations spaced at about 5 nm; there appeared to be several layers with a different orientation of the fibres in each. The outer coat was polyhedral with slightly concave pentagonal and hexagonal faces. A combination of geometrical considerations, analysis of the patterns of faces seen in the replicas and freeze-etchings, and model building suggested that the polyhedron had 12 pentagonal and approximately 12 hexagonal faces.