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The extinct organic-walled dinoflagellate cyst Duosphaeridium rugosum was first described from the Paleocene (Danian) of the southern USA. It has since been documented at several other localities around the world. Well-preserved specimens from the Danian of the Dakhla Formation in the Dababiya Corehole in southern Egypt, illustrated by light and scanning electron microscope images, reveal new morphological details, most notably parasutural features revealing the paratabulation. Detailed observations indicate that Duosphaeridium rugosum has an L-type ventral arrangement, an essentially straight sulcus, and apparently dextral torsion indicating a tentative affinity with the gonyaulacacean subfamily Cribroperidinioideae. Duosphaeridium rugosum occurs in an assemblage with many Danian markers; such as Danea californica, Carpatella cornuta and Lanternosphaeridium reinhardtii. Its earliest or first occurrence is contemporaneous with the upper part of nannoplankton zone NP2 and the uppermost part of the planktonic foraminiferal Parasubbotina pseudobulloides (P1a) zone. Its first occurrence is thus a useful stratigraphical marker for the Danian (earliest Paleocene). Duosphaeridium rugosum is associated with relatively abundant dinoflagellate cyst taxa such as Spiniferites, Achomosphaera, Fibrocysta, Pterodinium, Cordosphaeridium and Exochosphaeridium, which characterize neritic marine settings. [ABSTRACT FROM AUTHOR]