SUMMARYA spontaneous mutant of the homothallic pyrenomycete,Gelasinospora calospora, which produces small cleistothecial bodies each containing 1–5 asci with viable ascospores, is described. When perithecial and cleistothecial strains are paired, perithecia are formed which yield ascospores that segregate in essentially a 1:1 ratio for the ascocarp characters. The significance of this occurrence, relative to pyrenomycete systematics, is discussed.