The ultrastructure of the dimorphic ascospores of Mamillisphaeria dimorphospora are illustrated. The small brown ascospores differ from the larger hyaline ascospores in having a thick melanin layer in the ascospore wall. They are also 2–3 times smaller and do not germinate on PDA. The significance of these observations with respect to dispersal of aquatic fungi is discussed. The large hyaline ascospores may be morphologically adapted to dispersal in streams, while the smaller brown ascospores are thought to be adapted to aerial dispersal. The reasons for this are discussed. The method by which two spore types can be produced in separate asci in one ascoma is hypothesised.