Bioerosion was a common process affecting corals and stromatoporoids in reef and off-reef facies on the carbonate ramp that spanned the Ordovician–Silurian boundary on Anticosti Island. The probable worm boring Trypanites was the dominant macroboring, penetrating more than 40% of 2,500 massive tabulate corals and stromatoporoids examined, occasionally in dense concentrations. The frequency of macroboring was influenced by conditions at the facies level reflected by changes in grain size, water depth, storm reworking of sediments, and the nature of the skeletal mass bored. These factors regulated exposure time of the host-substrate surface to the watermass and thus influenced bioerosion. Bored specimens are most common in muddy off-reef facies, moderate in sandy off-reef facies, and less common in reefs. In off-reef facies, storm-enhanced deposition and reworking of sediments were most important in the burial of eligible host substrates. In reefs, the high competition for space by encrusting epizoans, combined with sedimentation, limited macroborers that preferred to excavate dead skeletal substrates. Skeletal density was the most important property of the host substrate in controlling boring frequency. Macroborers favored a dense host skeleton likely for its enhanced mechanical strength and adaptability for unlined borings, despite requiring greater energy for excavation. High-relief host skeletons were bored more frequently than tabular forms, since their greater capacity to shed sediment would have resulted in more prolonged exposure above the seafloor. The probable bivalve boring Petroxestes pera is rare. Temporal changes in boring frequency appear to reflect local shifts in facies and relative sea level. Mass-extinction events near the O/S boundary, which eliminated some host corals and stromatoporoids, had no apparent effect on boring frequency.