Upper Ordovician (Richmondian) patch reefs in exposures of the Meaford Formation (Streetsville Member) in Mullett Creek, Mississauga, Ontario contain a preserved coelobiontic (cavity-dwelling) biota. This is the first report of Upper Ordovician reef coelobionts.The cavity biota is dominated by algae that grew attached to the roof and walls of cavities. The fauna and flora consist of: (1) algae (including Girvanella, mammillary and laminated algae, micrite pendants, and a Renalcis-like alga); (2) calcareous vermiform tubes; (3) endolithic metazoa (Trypanites); (4) bryozoans; and (5) hemispherical worm tubes. In addition, thin micrite sheets, of possible organic origin, also line the roofs of some cavities.An infauna of burrowing worms, and coelobiontic sponges, which are common in Lower Cambrian and Middle Ordovician reef cavities, is absent from these Upper Ordovician cavities. In comparison, therefore, with older and also modern reef cavity biotas this Upper Ordovician coelobiontic biota is unusual.