ABSTRACTUltrastructural aspects of development of hat-shaped ascospores and centrum organization inO. cucullatumwere studied and compared with other species inCeratocystis sensu latoapparently having similar ascospores. The ascospore wall consisted of a reniform primary and ornamented secondary wall. Secondary wall ornamentation incorporated three equally spaced and parallel ridges along the length of the ascospore that gave the ascospore a triangular outline in transverse section. Brim appendages developed on the secondary wall at the rounded ends of the primary wall resulting in the hat-shaped appearance of the ascospores in side view. These appendages differed considerably from those of the symmetrically shaped and hemispherical bowler-hat ascospores previously observed inC. moniliformisandC. fimbriata.Centrum organization inO. cucullatumwas, however, similar to that previously illustrated ofO. davidsonii.In both of the latter species, young asci were arranged in a cluster at the ascomatal base. This was different to that inC. moniliformiswhere young asci lined the inner ascomatal wall.