Cultural, cytological, and developmental aspects of the patellariaceous ascomycete Rhytidhysterium rufulum are investigated. A comparison of the ultrastructural features of the R. rufulum ascus with those of other loculoascomycetes is made. Meiosis and mitosis in the ascus and ascospores are not significantly different from those considered typical for most Ascomycetes. Ascospore delimitation begins after the ascus reaches its 16-nucleate stage. Differences in centrum development and hymenial configuration indicate that the Patellariaceae and Hysteriaceae may have to be placed in different orders. The centrum of R. rufulum appears unique and characteristic, not of the Pleospora type.