The ascocarp is usually an ostiolate, flask-shaped perithecium. The internal tissues of the ascocarp comprise the centrum, and in ostiolate species the ostiole is lined with slender periphyses. The asci are variable in shape and usually contain eight ascospores. In most genera the asci are persistent, with a characteristic apical apparatus through which the ascospores are forcibly discharged. Members of the pyrenomycetes occur in a variety of habitats, but are especially common on dead wood, herbaceous stems, and in soils.
Many species are important pathogens of economic crops.
The centrum in young ascocarps contains lateral paraphyses. The asci are evanescent and the one-celled dark brown ascospores are extruded in a cirrhus. Most species are cellulose decomposers. There is a single family Chaetomiaceae. Chaetomium globosum - This is one of the most common species and has wavy hairs on its perithecia.