Bioerosion is a process with a high socioeconomic impact that contributes to coastal retreat, and likely to increase with climate change. Whereas limestone bioerosion is well explained by a combination of mechanical and chemical pathways, the bioerosion mechanisms of silicates, which are harder and chemically more resistant, remain elusive. Here we investigated the interface between siltstone and freshwater rock-boring bivalves Lignopholas fluminalis (Bivalvia: pholadidae). Remains of a microbial biofilm were observed only in the poorly consolidated part of the rock within the macroborings created by bivalves. Secondary Mn-bearing minerals identified in the biofilm suggest that microbes promoted silicate rock weathering by dissolving Mn-rich chlorites. Moreover, hard mineral debris found in a biofilm attached to the shells likely contributed to the abrasion of the rock substrate. thus, beyond the classical view of chemical and/or mechanical action(s) of macroborers, silicate bioerosion may also be facilitated by an unexpected synergistic association between macro-and microorganisms. Bioerosion is a commonplace strategy developed by living organisms, which consists in boring hard substrates of various origins, including biological materials (e.g., wood, shells, and bones) 1 , mud 2 , rocks 3 and even synthetic materials. Depending on the nature of the substrate and the borer, bioerosion ensures a wide range of metabolic activities and ecosystem services, ranging from nutrition 4 to the creation of microhabitats protected from predators for themselves as well as for secondary dwellers 5. Gaining knowledge into the occurrence, rates and mechanisms of boring is of fundamental importance for a series of reasons. First and historically, mankind has been confronted with macroborers through the damages caused by shipworms on vessels, wooden wharfs and docks 5,6. More broadly, bioerosion has socioeconomic impacts whenever manufactured materials are damaged, including plastic, metals and concrete materials such as levees or coastal defences 3,6. Second, bioerosion contributes to element recycling, shaping landscapes through the weakening of rocky shorelines and participating to coastal retreat 5 , for which the current rates are likely to be modified drastically as a result of climate change 6. Third, the creation of microhabitats by macroborers such as bivalves is correlated with a significant increase of the abundance of species assemblages, thus partly contributing to local faunal biodiversity 7. Finally, fossil records of macro-bioerosion may be used as a biological proxy to estimate the paleo-location of intertidal and shallow subtidal marine environments, marking ancient shorelines 8. The mechanisms of rock bioerosion associated to macroborers and especially bivalves have been a source of lively debate for decades, and can be schematically divided into two main pathways. First, rock boring can open