Bryozoan mounds, dominated by Batostoma chazyensis from the Laval formation of the Chazy Group (early Middle Ordovician) near Montreal, Quebec contain framework cavities that house a coelobiontic (cavity-dwelling) biota. Reports of coelobionts of Ordovician age are rare, and this is the first report of Chazy Group mounds containing coelobionts.Algae, sponges, and bryozoa (Batostoma) dominate the cavity biota, and are found encrusting cavity walls and roofs. The algae are represented by mammillary encrustations (now micrite), and Solenpora sp. on the roofs. The sponges are not identified, but occur both as isolated monaxial, triaxial, and six-rayed spicules, and as body fossils within the cavities. Coccoid cells, of unknown affinities, are also found. These cavities provide the first evidence of coelobiontic biological erosion. Trypanites, a compound morphogenus, is the only macroborer and is found in cavity walls; endolithic (boring) algae are present, and indicate that as early as the Middle Ordovician bio-erosion of reef and mound interior cavities was active and that micro- and macroendoliths had invaded the cavity habitat.